Medical Pharmacology: Chapter 38 — Antiparasitic Drugs -- Module 4 — Ectoparasiticides and Special Populations

Chapter 38 — Antiparasitic Drugs — Module 4 — Ectoparasiticides and Special Populations

1. [CASE 1 — QUESTION 1] A 49-year-old man who underwent renal transplantation 8 months ago and remains on tacrolimus and prednisone presents with widespread thick, scaly, hyperkeratotic plaques on his hands, feet, elbows, and scalp. He reports months of worsening scaling with surprisingly little itch. Skin scrapings reveal innumerable mites and eggs. Several staff on his ward have recently developed an itchy papular rash. Which diagnosis best fits this presentation?

A) Common scabies, with the typical burden of 10 to 15 mites
B) Contact dermatitis from a topical product, unrelated to any parasite
C) Crusted (Norwegian) scabies, a hyperinfestation with millions of mites that occurs in immunocompromised or neurologically impaired patients
D) Atopic eczema with secondary bacterial infection
E) Psoriasis vulgaris with no infectious component

ANSWER: C

Rationale:

The combination of an immunocompromised host (renal transplant on tacrolimus and prednisone), thick hyperkeratotic plaques teeming with mites, paradoxically little itch (impaired immune and scratch response), and secondary cases among ward staff is classic for crusted (Norwegian) scabies. This is a hyperinfestation state in which the mite burden reaches the millions rather than the 10 to 15 mites of common scabies, and it occurs specifically in immunocompromised or neurologically impaired patients.

Option A: Option A is incorrect: common scabies carries a small mite burden (10 to 15 mites) with intense itch and discrete burrows, not innumerable mites in hyperkeratotic crusts.
Option B: Option B is incorrect: contact dermatitis would not show innumerable mites on scraping or cause an outbreak among staff.
Option D: Option D is incorrect: atopic eczema does not produce mite-laden hyperkeratotic crusts or an institutional outbreak of an itchy rash.
Option E: Option E is incorrect: psoriasis is non-infectious and would not reveal mites on scraping or transmit a rash to staff.

2. [CASE 1 — QUESTION 2] Continuing with the same patient. A single application of permethrin 5% cream is tried but produces no meaningful improvement, and live mites persist. Which factor best explains why topical permethrin alone fails in this form of the disease?

A) The thick hyperkeratotic crust physically blocks penetration of the topical drug, so it cannot reach the enormous mite burden harbored within the crust
B) Permethrin has no activity against Sarcoptes scabiei in any form of scabies
C) Permethrin is inactivated by sunlight before it can work
D) The mites in crusted scabies are a different species that permethrin cannot kill
E) Permethrin must be taken orally, so the topical route is always ineffective

ANSWER: A

Rationale:

In crusted scabies the mites are sheltered within thick hyperkeratotic crusts. Topical permethrin cannot penetrate this crust adequately, so it fails to reach the millions of mites within, which is why single-agent topical therapy is inadequate and the crust must be addressed directly. This penetration barrier, combined with the massive mite burden, is the core reason monotherapy fails.

Option B: Option B is incorrect: permethrin is effective against Sarcoptes scabiei in common scabies; the problem here is penetration, not lack of intrinsic activity.
Option C: Option C is incorrect: permethrin is not inactivated by sunlight; the failure is due to the crust barrier.
Option D: Option D is incorrect: crusted scabies is caused by the same Sarcoptes scabiei mite, not a drug-resistant different species.
Option E: Option E is incorrect: permethrin is a topical agent that works well on accessible skin; the issue is the crust blocking access, not the topical route itself.

3. [CASE 1 — QUESTION 3] Continuing with the same patient. Recognizing that single-agent topical therapy is inadequate, the dermatology team designs a definitive regimen. Which approach is most appropriate for crusted scabies?

A) A second identical single application of permethrin 5% cream, with nothing else added
B) Oral metronidazole as monotherapy for 14 days
C) A topical corticosteroid alone to soften the plaques
D) Repeated courses of oral ivermectin combined with topical permethrin and a keratolytic agent (such as salicylic acid) to dissolve the crust and allow drug penetration, with contact precautions and treatment of close contacts
E) Reassurance and observation, since crusted scabies resolves on its own

ANSWER: D

Rationale:

Crusted scabies requires a multi-pronged regimen that addresses the mite burden, the crust barrier, and contagion at once. Repeated oral ivermectin reaches mites systemically that topical drug cannot, topical permethrin treats the surface, and a keratolytic such as salicylic acid dissolves the crust so the topical agent can finally penetrate. Strict contact precautions and simultaneous treatment of close contacts are essential because these patients are highly contagious and a source of outbreaks.

Option A: Option A is incorrect: repeating a single permethrin application will fail again because the crust still blocks penetration.
Option B: Option B is incorrect: metronidazole has no activity against the scabies mite and is not a treatment for crusted scabies.
Option C: Option C is incorrect: a corticosteroid is not scabicidal and may worsen the infestation by further suppressing local immunity.
Option E: Option E is incorrect: crusted scabies does not self-resolve; untreated it is severe, contagious, and a source of institutional outbreaks.

4. [CASE 1 — QUESTION 4] Continuing with the same patient. The infection-control team must prevent further ward transmission. Compared with common scabies, what environmental feature of crusted scabies most directly justifies aggressive decontamination of bedding, clothing, and furniture?

A) The mite cannot survive off the host at all, so only the patient needs treatment
B) The mite can survive off the host for up to about 72 hours in crusted scabies (versus roughly 24 to 36 hours in common scabies), so contaminated bedding, clothing, and surfaces can reseed the patient or infect others and require decontamination
C) The mite survives off the host for several months, making the environment permanently uninhabitable
D) Crusted scabies is not contagious, so no environmental measures are needed
E) The mite spreads only through the bloodstream, so surfaces are irrelevant

ANSWER: B

Rationale:

In crusted scabies the mite can survive off the host for up to about 72 hours, longer than the roughly 24 to 36 hours seen in common scabies, and the mite burden is enormous. Together these mean that bedding, clothing, and surfaces can remain infested and reseed the patient or transmit to others, which is exactly why aggressive environmental decontamination plus contact precautions are required alongside drug therapy in this highly contagious condition.

Option A: Option A is incorrect: the mite does survive off the host (up to about 72 hours here), so environmental measures are essential, not optional.
Option C: Option C is incorrect: survival is on the order of days (about 72 hours), not months, and the environment is not rendered permanently uninhabitable.
Option D: Option D is incorrect: crusted scabies is in fact highly contagious and a frequent source of institutional outbreaks.
Option E: Option E is incorrect: scabies spreads by skin contact and contaminated fomites, not through the bloodstream, so surfaces are highly relevant.

5. [CASE 2 — QUESTION 1] A 7-year-old girl has had head lice for 5 weeks. Her mother has applied permethrin 1% creme rinse exactly as directed on two occasions 9 days apart, with careful technique and nit combing. Live, moving lice are still seen at the scalp. The family lives in an area where surveillance shows the knockdown resistance (kdr) sodium-channel mutation in the majority of local head lice. What is the most likely reason for the persistent infestation?

A) The lice were never present, and the mother is mistaken about live lice
B) The second application was unnecessary and caused the lice to multiply
C) Permethrin is ovicidal, so the eggs were killed and the live lice must be a new infestation acquired today
D) The child has developed an allergy to permethrin that prevents it from working
E) Pyrethroid (permethrin) treatment failure from kdr-mediated target-site resistance, which is prevalent in this region and confers cross-resistance to pyrethroids regardless of correct application

ANSWER: E

Rationale:

Two correctly applied permethrin courses with good technique, in a region where the kdr mutation is prevalent, point to pyrethroid treatment failure from target-site resistance. The kdr mutation alters the sodium-channel binding site, reducing permethrin binding and conferring cross-resistance across the pyrethroid class; correct application cannot overcome it. Persistent live lice after proper treatment in a high-kdr area is the classic clinical signature of resistance.

Option A: Option A is incorrect: live, moving lice seen at the scalp confirm an active infestation; the failure is real, not a misperception.
Option B: Option B is incorrect: the second application is a standard required step (permethrin is not reliably ovicidal) and does not cause lice to multiply.
Option C: Option C is incorrect: permethrin is in fact not reliably ovicidal, which is why a second dose is needed; the persistence reflects resistance, not necessarily a brand-new infestation.
Option D: Option D is incorrect: a drug allergy would cause a hypersensitivity reaction, not loss of pediculicidal efficacy; the failure is due to resistance.

6. [CASE 2 — QUESTION 2] Continuing with the same patient. The mother asks whether simply using more permethrin, or a stronger pyrethroid, would solve the problem. Which explanation of the resistance mechanism is correct?

A) Resistance is caused by the lice metabolizing permethrin faster, so a higher dose will overwhelm the enzyme and work
B) kdr is a point mutation in the louse voltage-gated sodium channel that lowers permethrin binding affinity at the target itself, so adding more drug or switching to another pyrethroid still binds poorly to the altered channel and cannot overcome the resistance
C) Resistance is due to a thicker cuticle, so warming the scalp will let the drug through
D) kdr makes the lice immune to all insecticides of every mechanism, so no treatment can work
E) Resistance occurs only against the eggs, not the adult lice, so a longer contact time will cure it

ANSWER: B

Rationale:

kdr is a target-site resistance: a point mutation (for example T929I or L932F) in the voltage-gated sodium channel reduces permethrin binding affinity at the channel itself. Because the defect is in how the drug fits its target, neither a higher dose nor a different pyrethroid restores efficacy, since all pyrethroids share that channel target and thus the cross-resistance. The solution is to switch to an agent with a different mechanism, not to escalate the dose.

Option A: Option A is incorrect: kdr is a target-site mutation, not a metabolic one, so the saturate-the-enzyme logic does not apply.
Option C: Option C is incorrect: kdr is a sodium-channel binding-site change, not a cuticular thickening, and warming the scalp does not reverse it.
Option D: Option D is incorrect: kdr confers cross-resistance only across pyrethroids (sodium-channel agents); off-channel agents such as spinosad and malathion remain effective.
Option E: Option E is incorrect: kdr affects the sodium-channel target in lice generally and is not an egg-only phenomenon overcome by longer contact time.

7. [CASE 2 — QUESTION 3] Continuing with the same patient. The clinician decides to switch agents. Which choice is the most rational next step, and why?

A) Spinosad 0.9% suspension, because it activates nicotinic acetylcholine receptors and glutamate-gated chloride channels, a mechanism off the sodium channel that remains effective against kdr-resistant lice
B) A third course of permethrin 1% at a higher concentration
C) Another pyrethrin-based over-the-counter shampoo
D) An oral antihistamine as the definitive cure
E) No further treatment, accepting the infestation as untreatable

ANSWER: A

Rationale:

Because kdr confers cross-resistance to all pyrethroids regardless of dose, the rational move is an agent with a different mechanism. Spinosad activates nicotinic acetylcholine receptors and glutamate-gated chloride channels, a target entirely off the sodium channel, so it retains full activity against kdr-resistant lice. (Malathion and benzyl alcohol would be equally rational off-channel alternatives.)

Option B: Option B is incorrect: a higher permethrin concentration cannot overcome a target-site mutation; the binding site itself is altered.
Option C: Option C is incorrect: pyrethrin shampoos are pyrethroid-based and share kdr cross-resistance, and are less effective than permethrin against resistant lice.
Option D: Option D is incorrect: an antihistamine may relieve itch but does not kill lice and is not a cure for the infestation.
Option E: Option E is incorrect: the infestation is readily curable with an off-channel agent such as spinosad, so abandoning treatment is unjustified.

8. [CASE 2 — QUESTION 4] Continuing with the same patient. The mother notes that permethrin required two applications and asks whether spinosad will also need a mandatory second treatment a week later. What is the best answer, and the reason?

A) Spinosad always requires three applications because it is weaker than permethrin
B) Spinosad must be left on for 12 hours and reapplied daily for a week
C) Spinosad cannot kill lice at all without a second dose
D) A single spinosad application is usually sufficient because spinosad is ovicidal (it kills the eggs as well as the live lice), so unlike permethrin it does not depend on a mandatory second treatment to catch newly hatched lice
E) Spinosad is taken orally, so the question of a second topical dose does not apply

ANSWER: D

Rationale:

Permethrin needs a second application about a week later because it is not reliably ovicidal, so a repeat dose is required to kill lice hatching from surviving eggs. Spinosad has both pediculicidal and ovicidal activity, killing eggs as well as adults, so a single application is usually sufficient and a mandatory second treatment is generally not needed. The egg-killing property is the operative difference the mother is asking about.

Option A: Option A is incorrect: spinosad does not require three applications and is not weaker than permethrin against resistant lice; a single application usually suffices.
Option B: Option B is incorrect: spinosad is applied for about 10 minutes, not 12 hours, and is not reapplied daily for a week.
Option C: Option C is incorrect: spinosad effectively kills lice with a single application; it does not require a second dose to work.
Option E: Option E is incorrect: spinosad 0.9% is a topical suspension, not an oral drug; its single-application efficacy comes from killing eggs.

9. [CASE 3 — QUESTION 1] A 31-year-old woman who is 18 weeks pregnant presents with intensely itchy burrows in the finger webs and on the wrists, and a household contact was recently treated for scabies. She is otherwise healthy and asks for a treatment that is safe in pregnancy. Which agent is preferred for her scabies?

A) Oral ivermectin as the routine first-line choice for scabies in pregnancy
B) Lindane lotion, which is considered safe in pregnancy
C) Permethrin 5% cream, the preferred scabicide in pregnancy because its dermal absorption is under 2 percent, so very little drug reaches the maternal circulation or the fetus
D) Diethylcarbamazine, which is indicated for scabies in pregnancy
E) No treatment until after delivery, since scabies is never treated in pregnancy

ANSWER: C

Rationale:

Permethrin 5% cream is the preferred scabicide in pregnancy. Its dermal absorption is below 2 percent, so minimal drug reaches the maternal circulation or fetus, and it has a long record of safe use. Scabies should be treated in pregnancy, and permethrin is the agent of choice for this patient.

Option A: Option A is incorrect: ivermectin has limited pregnancy safety data and is generally avoided for elective use, reserved for life-threatening indications; it is not the routine first choice.
Option B: Option B is incorrect: lindane is contraindicated in pregnancy because of significant central nervous system absorption and neurotoxicity.
Option D: Option D is incorrect: diethylcarbamazine (DEC) treats lymphatic filariasis and is contraindicated in pregnancy; it is not a scabies treatment.
Option E: Option E is incorrect: scabies can and should be treated in pregnancy with a safe agent such as permethrin; withholding treatment is inappropriate.

10. [CASE 3 — QUESTION 2] Continuing with the same patient. A relative offers her a bottle of lindane lotion left over from a prior infestation and suggests she use it instead. Why should she avoid lindane during pregnancy?

A) Lindane is ineffective against the scabies mite in everyone
B) Lindane is safe in pregnancy but simply more expensive than permethrin
C) Lindane is an antimalarial with no scabicidal activity
D) Lindane is preferred in pregnancy because it is poorly absorbed
E) Lindane is contraindicated in pregnancy because it is absorbed through the skin and its central nervous system toxicity (via GABA-A receptor blockade) poses a neurotoxic risk

ANSWER: E

Rationale:

Lindane is contraindicated in pregnancy because it undergoes significant dermal absorption, and its mechanism, blockade of GABA-A receptors (the receptors for the main inhibitory neurotransmitter), produces central nervous system (CNS) neurotoxicity. The systemic exposure and neurotoxic potential make it inappropriate in pregnancy, and it has been banned in many countries. Permethrin, with its very low absorption, is the preferred agent instead.

Option A: Option A is incorrect: lindane does have scabicidal activity; it is avoided because of toxicity, not lack of efficacy.
Option B: Option B is incorrect: lindane is not safe in pregnancy; the issue is neurotoxicity from absorption, not cost.
Option C: Option C is incorrect: lindane is a scabicide and pediculicide, not an antimalarial.
Option D: Option D is incorrect: lindane is in fact appreciably absorbed through the skin, which is the basis for its contraindication, so it is not preferred in pregnancy.

11. [CASE 3 — QUESTION 3] Continuing with the same patient. She also has a confirmed soil-transmitted helminth infection (hookworm) contributing to anemia, and she lives in a high-prevalence region. Regarding anthelmintic treatment of this infection now, in her second trimester, what is the most appropriate position?

A) Anthelmintics are absolutely forbidden at every stage of pregnancy without exception
B) A single dose of albendazole or mebendazole is considered acceptable from the second trimester in high-prevalence settings, because the benefit of treating hookworm (reducing anemia and adverse pregnancy outcomes) is judged to exceed the drug risk
C) She should be given lindane to treat the hookworm
D) She should receive diethylcarbamazine, the preferred drug for hookworm in pregnancy
E) Hookworm should be treated only with intravenous therapy in pregnancy

ANSWER: B

Rationale:

For soil-transmitted helminths in pregnancy, a single dose of albendazole or mebendazole is considered acceptable from the second trimester in high-prevalence settings, reflecting a risk-benefit judgment: untreated hookworm contributes to maternal anemia and adverse pregnancy outcomes, and that harm is judged to exceed the drug risk after the first trimester. This is the basis for including pregnant women (after the first trimester) in deworming programs in endemic areas.

Option A: Option A is incorrect: anthelmintics are not absolutely forbidden throughout pregnancy; single-dose benzimidazoles are accepted from the second trimester in high-prevalence settings.
Option C: Option C is incorrect: lindane is an ectoparasiticide, not an anthelmintic, and is contraindicated in pregnancy.
Option D: Option D is incorrect: diethylcarbamazine is for lymphatic filariasis, is contraindicated in pregnancy, and is not the drug for hookworm.
Option E: Option E is incorrect: soil-transmitted helminths are treated with oral single-dose benzimidazoles, not intravenous therapy.

12. [CASE 3 — QUESTION 4] Continuing with the same patient. Months earlier, before she was pregnant, she had been screened for lymphatic filariasis during travel. If filariasis treatment were ever considered, which statement about diethylcarbamazine (DEC) in pregnancy is correct?

A) Diethylcarbamazine is contraindicated in pregnancy and should not be used; treatment is deferred until after delivery
B) Diethylcarbamazine is the preferred first-line agent in every trimester of pregnancy
C) Diethylcarbamazine is a topical scabicide that is safe in pregnancy
D) Diethylcarbamazine is identical to permethrin and is preferred in pregnancy
E) Diethylcarbamazine must be given together with lindane in pregnancy for full effect

ANSWER: A

Rationale:

Diethylcarbamazine (DEC), used for lymphatic filariasis, is contraindicated in pregnancy because of fetal harm seen in animal studies; treatment is generally deferred until after delivery. Recognizing DEC as an agent to avoid in pregnancy is the key point, in contrast to permethrin (preferred for scabies) and the conditional acceptability of single-dose benzimidazoles after the first trimester.

Option B: Option B is incorrect: DEC is contraindicated in pregnancy, not a preferred agent in any trimester.
Option C: Option C is incorrect: DEC is a systemic antifilarial drug, not a topical scabicide, and it is not safe in pregnancy.
Option D: Option D is incorrect: DEC is not identical to permethrin and is not preferred in pregnancy; permethrin is the low-absorption scabicide, DEC is a contraindicated antifilarial.
Option E: Option E is incorrect: DEC is not combined with lindane, and lindane is itself contraindicated in pregnancy.

13. [CASE 4 — QUESTION 1] A 44-year-old man with AIDS (acquired immunodeficiency syndrome) is diagnosed with cerebral toxoplasmosis and started on pyrimethamine plus sulfadiazine. The team reviews how pyrimethamine works against the parasite. What is pyrimethamine's mechanism of action?

A) It prolongs the open state of voltage-gated sodium channels
B) It physically blocks the parasite's breathing pores
C) It inhibits acetylcholinesterase, causing acetylcholine to accumulate
D) It inhibits dihydrofolate reductase (DHFR), the enzyme that regenerates the active form of folate required for DNA synthesis, thereby blocking folate-dependent nucleic acid production
E) It opens glutamate-gated chloride channels

ANSWER: D

Rationale:

Pyrimethamine is an antifolate that inhibits dihydrofolate reductase (DHFR), the enzyme that regenerates tetrahydrofolate (the active folate) needed for DNA synthesis. By blocking folate-dependent nucleic acid production, it impairs parasite replication. The same enzyme exists in human cells, which is the basis for its hematologic toxicity and the need for folinic acid rescue.

Option A: Option A is incorrect: prolonging sodium-channel opening is the pyrethroid (permethrin) mechanism, not pyrimethamine.
Option B: Option B is incorrect: physical blockade of breathing pores is the benzyl alcohol mechanism, not pyrimethamine.
Option C: Option C is incorrect: acetylcholinesterase inhibition is the malathion mechanism, not pyrimethamine.
Option E: Option E is incorrect: opening glutamate-gated chloride channels is the ivermectin mechanism, not pyrimethamine.

14. [CASE 4 — QUESTION 2] Continuing with the same patient. After 10 days he develops fatigue, easy bruising, and laboratory evidence of falling white-cell count, anemia, and thrombocytopenia. The team realizes a recommended supportive medication was never started. What is the correct action?

A) Stop all antiparasitic therapy permanently and give no further treatment for toxoplasmosis
B) Add iron supplementation, since the anemia is from iron deficiency
C) Start folinic acid (leucovorin) to rescue the human bone marrow from pyrimethamine's antifolate effect, since folinic acid supplies a usable folate that human cells can use without dihydrofolate reductase, and monitor the complete blood count
D) Begin a proton pump inhibitor to protect the marrow
E) Increase the pyrimethamine dose to push past the cytopenias

ANSWER: C

Rationale:

Pyrimethamine's inhibition of dihydrofolate reductase (DHFR) suppresses human bone marrow, causing megaloblastic anemia, leukopenia, and thrombocytopenia. Folinic acid (leucovorin) provides a reduced folate that human cells can use without needing DHFR, rescuing the marrow while leaving the antiparasitic effect intact. It is mandatory during pyrimethamine therapy and was omitted here, so it should be started now, with complete blood count (CBC) monitoring.

Option A: Option A is incorrect: cerebral toxoplasmosis still needs treatment; the answer is to add folinic acid rescue, not to abandon therapy.
Option B: Option B is incorrect: the anemia is megaloblastic (folate-related), not iron deficiency, so iron supplementation does not address the mechanism.
Option D: Option D is incorrect: a proton pump inhibitor protects the gastric mucosa, not the bone marrow, and does nothing for antifolate toxicity.
Option E: Option E is incorrect: raising the pyrimethamine dose would worsen the marrow suppression, not resolve it.

15. [CASE 4 — QUESTION 3] Continuing with the same patient. Folinic acid is started and the regimen continues. What laboratory monitoring is most appropriate during acute pyrimethamine therapy for toxoplasmosis?

A) A complete blood count checked weekly during acute treatment, to detect bone marrow suppression early
B) A bone density scan every week
C) Thyroid function tests daily
D) No laboratory monitoring is needed once folinic acid has been started
E) Continuous cardiac telemetry for QTc monitoring

ANSWER: A

Rationale:

Because pyrimethamine suppresses the bone marrow through dihydrofolate reductase (DHFR) inhibition, a complete blood count (CBC) is monitored weekly during acute treatment to detect leukopenia, anemia, or thrombocytopenia early, even with folinic acid co-administration. This hematologic surveillance is the appropriate monitoring for this antifolate regimen.

Option B: Option B is incorrect: pyrimethamine does not cause bone loss, so a bone density scan is not the relevant monitoring.
Option C: Option C is incorrect: pyrimethamine toxicity is hematologic, not thyroid, so daily thyroid testing is not indicated.
Option D: Option D is incorrect: folinic acid reduces but does not eliminate the risk of marrow suppression, so CBC monitoring is still required.
Option E: Option E is incorrect: pyrimethamine is not a notable QTc-prolonging drug; the monitoring concern is the blood count, not the QTc.

16. [CASE 4 — QUESTION 4] Continuing with the same patient. A trainee asks how to characterize the anemia that pyrimethamine produces and how it differs mechanistically from iron-deficiency anemia. Which statement is correct?

A) Pyrimethamine causes a microcytic, iron-deficiency anemia best treated with iron
B) Pyrimethamine causes hemolytic anemia from oxidative red-cell destruction
C) Pyrimethamine causes anemia only by gastrointestinal blood loss
D) Pyrimethamine causes aplastic anemia exclusively, with no relation to folate
E) Pyrimethamine causes a megaloblastic anemia by blocking folate regeneration (dihydrofolate reductase inhibition), impairing DNA synthesis in red-cell precursors; this is mechanistically distinct from iron-deficiency anemia, which results from inadequate iron for hemoglobin and responds to iron rather than to folate rescue

ANSWER: E

Rationale:

Pyrimethamine produces a megaloblastic anemia: by inhibiting dihydrofolate reductase (DHFR) it blocks regeneration of active folate, impairing DNA synthesis in red-cell precursors and yielding large, immature (megaloblastic) cells. This is mechanistically different from iron-deficiency anemia, where the problem is insufficient iron for hemoglobin synthesis; iron deficiency responds to iron, whereas the pyrimethamine-induced defect is addressed by folinic acid rescue, not iron. Distinguishing the folate-related mechanism from the iron-related one is the conceptual point.

Option A: Option A is incorrect: the pyrimethamine anemia is megaloblastic (folate-related), not a microcytic iron-deficiency anemia, and iron does not correct it.
Option B: Option B is incorrect: oxidative hemolysis describes primaquine in G6PD deficiency, not pyrimethamine's antifolate marrow effect.
Option C: Option C is incorrect: the anemia is due to impaired folate-dependent red-cell production, not gastrointestinal blood loss.
Option D: Option D is incorrect: the anemia is megaloblastic and directly related to folate-pathway blockade, not a folate-independent aplastic anemia.

17. [CASE 5 — QUESTION 1] A 38-year-old man from an endemic region presents with new-onset seizures, and imaging plus serology confirm neurocysticercosis (NCC) with viable parenchymal cysts. He is started on a prolonged course of albendazole. Which monitoring plan is most appropriate during extended albendazole therapy?

A) No laboratory monitoring is required, since albendazole has no systemic toxicity
B) Liver function tests and a complete blood count checked about every 2 weeks during each treatment cycle, because prolonged albendazole can cause transaminase elevation (in roughly 10 to 17 percent) and, in a minority, leukopenia and thrombocytopenia
C) Daily blood glucose checks, because albendazole commonly causes hyperglycemia
D) Weekly audiograms, because albendazole causes irreversible deafness
E) Continuous telemetry, because albendazole prolongs the QTc

ANSWER: B

Rationale:

Prolonged albendazole (as used for neurocysticercosis) can raise transaminases in approximately 10 to 17 percent of patients and, in a minority, cause leukopenia and thrombocytopenia. Accordingly, liver function tests (LFTs) and a complete blood count (CBC) are monitored about every 2 weeks during each treatment cycle to detect hepatotoxicity and marrow suppression early.

Option A: Option A is incorrect: prolonged albendazole does have monitorable hepatic and hematologic toxicity, so laboratory surveillance is required.
Option C: Option C is incorrect: albendazole is not a characteristic cause of hyperglycemia, so daily glucose checks are not the relevant monitoring.
Option D: Option D is incorrect: irreversible deafness is not an albendazole toxicity, and audiograms are not the appropriate surveillance.
Option E: Option E is incorrect: albendazole is not a notable QTc-prolonging drug; the monitoring concern is LFTs and CBC, not telemetry.

18. [CASE 5 — QUESTION 2] Continuing with the same patient. Because killing the cysts can provoke a brisk inflammatory reaction in the brain, the team adds a corticosteroid and is also considering praziquantel. What interaction between corticosteroids and praziquantel must be anticipated?

A) Corticosteroids raise praziquantel cerebrospinal fluid levels, risking neurotoxicity
B) Corticosteroids have no effect on praziquantel in any compartment
C) Corticosteroids block praziquantel absorption from the gut entirely
D) Corticosteroids reduce praziquantel penetration into the cerebrospinal fluid by roughly 50 percent, lowering drug exposure at the central nervous system parasites, which must be accounted for when both are used
E) Corticosteroids convert praziquantel into an inactive antimalarial compound

ANSWER: D

Rationale:

Corticosteroids, frequently needed in neurocysticercosis to control the inflammatory response to dying cysts, reduce praziquantel penetration into the cerebrospinal fluid (CSF) by approximately 50 percent. This lowers praziquantel exposure precisely at the site of the central nervous system parasites and must be accounted for when the two are co-administered. (By contrast, corticosteroids increase benznidazole plasma levels, a separate interaction.)

Option A: Option A is incorrect: corticosteroids lower, not raise, praziquantel CSF levels, so the concern is reduced efficacy rather than increased neurotoxicity.
Option B: Option B is incorrect: there is a clinically relevant effect; corticosteroids reduce praziquantel CSF concentrations by about half.
Option C: Option C is incorrect: the interaction is reduced CSF penetration, not complete blockade of gut absorption.
Option E: Option E is incorrect: corticosteroids do not transform praziquantel into an antimalarial compound; they reduce its CSF penetration.

19. [CASE 5 — QUESTION 3] Continuing with the same patient. He is newly found to have active pulmonary tuberculosis, and the team plans to start rifampicin while he is on praziquantel. The pharmacist raises a concern. What is the interaction and the appropriate response?

A) Rifampicin raises praziquantel to toxic levels, so praziquantel must be stopped at once
B) There is no interaction, since praziquantel is not hepatically metabolized
C) Rifampicin is a potent CYP3A4 inducer that accelerates praziquantel's metabolism and lowers its plasma levels by about 85 percent, risking treatment failure, so the two should not be co-administered and therapy must be re-planned
D) Rifampicin and praziquantel bind in the gut, so dosing them 2 hours apart fixes it
E) Rifampicin increases praziquantel absorption and improves its efficacy, so no change is needed

ANSWER: C

Rationale:

Rifampicin is a potent inducer of cytochrome P450 3A4 (CYP3A4), the enzyme that metabolizes praziquantel. Induction speeds praziquantel breakdown and lowers plasma levels by roughly 85 percent, which can cause treatment failure. Because the reduction is so large, the combination should be avoided and therapy re-planned (for example, sequencing or alternative management) rather than co-administered.

Option A: Option A is incorrect: enzyme induction lowers, not raises, praziquantel levels, so the risk is treatment failure, not toxicity from high levels.
Option B: Option B is incorrect: praziquantel is extensively metabolized by hepatic CYP3A4, which is why a CYP3A4 inducer affects it.
Option D: Option D is incorrect: the interaction is metabolic enzyme induction, not gut binding, so spacing the doses does not prevent the loss of efficacy.
Option E: Option E is incorrect: rifampicin reduces praziquantel exposure through faster metabolism; it does not improve efficacy.

20. [CASE 5 — QUESTION 4] Continuing with the same patient. Two weeks into albendazole therapy, routine labs show transaminases (liver enzymes) at about three times the upper limit of normal; he is asymptomatic. How should this finding be interpreted and managed?

A) This is the expected, usually mild-to-moderate and reversible transaminase elevation seen with prolonged albendazole; it is managed by closer monitoring and dose reduction or temporary discontinuation, with levels typically returning toward normal after the drug is reduced or stopped
B) This indicates fulminant hepatic failure requiring immediate liver transplantation regardless of symptoms
C) This is unrelated to albendazole and should be ignored without any change in monitoring
D) This proves the albendazole dose is too low and should be doubled
E) This represents hemolysis rather than a hepatic effect and requires G6PD testing

ANSWER: A

Rationale:

Prolonged albendazole causes transaminase elevation in roughly 10 to 17 percent of patients; most elevations are mild to moderate and reversible with dose reduction or discontinuation. An asymptomatic rise to about three times normal at the 2-week check is the expected pattern detected by routine monitoring and is managed by closer surveillance and reducing or temporarily stopping the drug, with levels generally normalizing afterward. This is precisely why LFT monitoring every 2 weeks is built into extended albendazole therapy.

Option B: Option B is incorrect: an asymptomatic moderate transaminase rise is not fulminant hepatic failure and does not call for transplantation; it is typically reversible.
Option C: Option C is incorrect: the elevation is a known albendazole effect and should prompt closer monitoring and possible dose adjustment, not be ignored.
Option D: Option D is incorrect: hepatotoxicity is dose-related, so doubling the dose would worsen it, not help.
Option E: Option E is incorrect: transaminase elevation reflects a hepatic effect of albendazole, not hemolysis; G6PD testing relates to primaquine, not this finding.

21. [CASE 6 — QUESTION 1] A 33-year-old man returns from sub-Saharan Africa and is being evaluated for malaria management and future prophylaxis. His history includes bipolar disorder with a prior psychiatric hospitalization and one adult seizure. A colleague suggests weekly mefloquine for future prophylaxis. Why is mefloquine inappropriate for this patient?

A) Mefloquine is a topical agent and cannot be used systemically
B) Mefloquine is contraindicated because it raises blood pressure dangerously
C) Mefloquine has no contraindications and is appropriate for everyone
D) Mefloquine is contraindicated only in patients with diabetes
E) Mefloquine carries a boxed neuropsychiatric warning and is contraindicated in patients with a history of psychiatric illness, a seizure disorder, or cardiac conduction abnormalities; this patient's bipolar disorder and prior seizure make it unsafe, so an alternative should be selected

ANSWER: E

Rationale:

Mefloquine has a boxed warning for neuropsychiatric adverse effects (anxiety, vivid nightmares, dizziness, insomnia, and, rarely, psychosis, seizures, and encephalopathy) and lowers the seizure threshold. It is contraindicated in patients with a history of psychiatric illness, seizure disorder, or cardiac conduction abnormalities. This patient's bipolar disorder and prior seizure place him squarely within those contraindications, so a different antimalarial should be chosen.

Option A: Option A is incorrect: mefloquine is an oral systemic antimalarial, not a topical agent.
Option B: Option B is incorrect: the relevant contraindication is neuropsychiatric and seizure-related, not hypertensive.
Option C: Option C is incorrect: mefloquine does have important contraindications, including psychiatric and seizure history.
Option D: Option D is incorrect: the contraindications relate to psychiatric illness, seizures, and cardiac conduction abnormalities, not specifically diabetes.

22. [CASE 6 — QUESTION 2] Continuing with the same patient. A consultant mentions that an older antimalarial, halofantrine, must never be used in a particular sequence. Which combination is specifically dangerous, and why?

A) Halofantrine after albendazole, because of additive marrow suppression
B) Halofantrine after mefloquine, because both prolong the QTc interval and the sequence can cause fatal QTc prolongation and torsades de pointes
C) Halofantrine after permethrin, because of additive sodium-channel toxicity
D) Halofantrine after metronidazole, because of additive bleeding
E) Halofantrine after pyrimethamine, because of additive hemolysis

ANSWER: B

Rationale:

Halofantrine markedly prolongs the QTc interval, and mefloquine also affects cardiac conduction. Giving halofantrine after mefloquine produces additive QTc prolongation that can precipitate torsades de pointes and sudden death. This dangerous cardiac interaction is a major reason halofantrine has largely fallen out of use, and it is especially relevant in a patient who may have received mefloquine.

Option A: Option A is incorrect: the defining halofantrine hazard is cardiac QTc prolongation with mefloquine, not additive marrow suppression with albendazole.
Option C: Option C is incorrect: halofantrine's danger is QTc prolongation, not a sodium-channel interaction with permethrin.
Option D: Option D is incorrect: the dangerous interaction is cardiac with mefloquine, not additive bleeding with metronidazole.
Option E: Option E is incorrect: halofantrine's signature risk is QTc prolongation after mefloquine, not hemolysis with pyrimethamine.

23. [CASE 6 — QUESTION 3] Continuing with the same patient. He is found to have severe Plasmodium falciparum malaria, and intravenous quinine is started in hospital. Which statement about quinine's clinical use and toxicity is correct?

A) Quinine is used only for uncomplicated malaria and is contraindicated in severe disease
B) Quinine has no recognized toxicity and requires no monitoring
C) Quinine is used for severe malaria (and for first-trimester malaria), dosed around 10 mg/kg every 8 hours, and at toxic levels produces cinchonism (tinnitus, visual disturbance, headache, altered mental status), along with dose-dependent QTc prolongation
D) Quinine is a topical scabicide with no antimalarial role
E) Quinine is given as a single lifelong dose and never affects cardiac conduction

ANSWER: C

Rationale:

Quinine remains important for severe malaria (given intravenously in hospital) and for first-trimester malaria when artemisinin-based therapy is not used, typically dosed around 10 mg/kg every 8 hours. Its characteristic toxicity at high levels is cinchonism, a cluster of tinnitus, visual disturbance, headache, and altered mental status, and it also causes dose-dependent QTc prolongation. These define its appropriate use and the monitoring it requires.

Option A: Option A is incorrect: quinine is in fact used for severe malaria, so restricting it to uncomplicated disease and calling it contraindicated in severe disease is wrong.
Option B: Option B is incorrect: quinine has well-recognized toxicity (cinchonism, QTc prolongation), so monitoring is required.
Option D: Option D is incorrect: quinine is a systemic antimalarial, not a topical scabicide.
Option E: Option E is incorrect: quinine is dosed repeatedly during a treatment course, not as a single lifelong dose, and it does affect cardiac conduction (QTc prolongation).

24. [CASE 6 — QUESTION 4] Continuing with the same patient. He recovers from the acute episode, and because mixed infection with Plasmodium vivax is identified, radical cure with primaquine is planned to clear the dormant liver forms. What must be done before starting primaquine?

A) Test glucose-6-phosphate dehydrogenase (G6PD) status first, because primaquine can trigger severe hemolysis (red-cell destruction) in G6PD-deficient patients
B) Obtain a bone density scan, because primaquine weakens bone
C) Start a proton pump inhibitor, because primaquine damages the stomach lining
D) Check a lipid panel, because primaquine raises cholesterol
E) No testing is required before primaquine under any circumstances

ANSWER: A

Rationale:

Primaquine imposes an oxidative load on red blood cells and can cause acute hemolytic anemia in patients deficient in glucose-6-phosphate dehydrogenase (G6PD), the enzyme that protects red cells from oxidative stress. For this reason, G6PD testing is mandatory before primaquine is given to any patient, so that deficient individuals can be identified and the hemolysis risk managed before radical-cure therapy proceeds.

Option B: Option B is incorrect: primaquine does not cause bone loss, so a bone density scan is not the required screen.
Option C: Option C is incorrect: a proton pump inhibitor protects the gastric mucosa but does nothing to prevent primaquine-induced hemolysis.
Option D: Option D is incorrect: primaquine's key toxicity is hemolysis in G6PD deficiency, not a lipid effect, so a lipid panel is not the relevant test.
Option E: Option E is incorrect: testing is required; giving primaquine without checking G6PD status risks severe hemolysis in a deficient patient.

25. [CASE 7 — QUESTION 1] A 52-year-old man with advanced HIV (human immunodeficiency virus) infection has disseminated strongyloidiasis and severe crusted scabies. The team plans oral ivermectin but is concerned about central nervous system safety. As background, why is ivermectin normally safe in the human central nervous system?

A) Ivermectin is destroyed in the stomach before it can reach the brain
B) Ivermectin cannot cross any biological membrane in humans
C) Humans lack the glutamate-gated chloride channels that ivermectin targets, but those channels are also absent from the brain, so location is irrelevant
D) Ivermectin is actively pumped out of the brain by P-glycoprotein, an efflux transporter at the blood-brain barrier, so under normal conditions it does not accumulate in the central nervous system
E) Ivermectin is converted in the liver into a compound that cannot enter the brain

ANSWER: D

Rationale:

Ivermectin is normally kept out of the central nervous system (CNS) by P-glycoprotein (P-gp), an efflux transporter at the blood-brain barrier (BBB) that actively pumps the drug back out of the brain. This is why ivermectin is safe at standard doses in people with an intact barrier: its selective antiparasitic target (glutamate-gated chloride channels in invertebrates) plus active exclusion from the human CNS together give the safety margin. Understanding this sets up why certain patients are at higher risk.

Option A: Option A is incorrect: ivermectin is absorbed and active after oral dosing; its CNS safety is due to P-gp efflux, not gastric destruction.
Option B: Option B is incorrect: ivermectin does cross membranes and is absorbed; the brain is specifically protected by P-gp efflux, not by a general inability to cross membranes.
Option C: Option C is incorrect: humans do have glutamate-gated chloride channels only in the CNS, but they are normally protected precisely because P-gp keeps ivermectin out; the channels are not simply absent everywhere.
Option E: Option E is incorrect: CNS exclusion is due to P-gp efflux at the barrier, not conversion to a brain-impermeable hepatic metabolite.

26. [CASE 7 — QUESTION 2] Continuing with the same patient. His antiretroviral regimen includes ritonavir, and he may have HIV-related encephalopathy. Why do these factors raise concern about ivermectin central nervous system toxicity?

A) Ritonavir destroys ivermectin in the gut, so the drug will be ineffective rather than toxic
B) Ritonavir inhibits P-glycoprotein, reducing the efflux that normally keeps ivermectin out of the brain, and HIV encephalopathy may compromise the blood-brain barrier; both factors converge to increase ivermectin's central nervous system penetration and the risk of neurotoxicity
C) Ritonavir and ivermectin cannot be given within the same calendar month for unrelated reasons
D) Ritonavir increases ivermectin absorption from the gut but has no effect on brain penetration
E) Ivermectin is unsafe in everyone, so the patient's HIV status is irrelevant

ANSWER: B

Rationale:

Two risk factors stack in this patient. Ritonavir is a potent inhibitor of P-glycoprotein (P-gp), the efflux pump that normally keeps ivermectin out of the central nervous system (CNS), so inhibiting it lowers that protection. In addition, HIV-related encephalopathy can compromise blood-brain barrier (BBB) integrity. Together, reduced P-gp efflux and a potentially leaky barrier converge to increase ivermectin CNS penetration and the risk of neurotoxicity, which is the basis for caution here.

Option A: Option A is incorrect: the concern is increased CNS penetration and toxicity, not gastric destruction rendering the drug ineffective.
Option C: Option C is incorrect: there is no blanket one-month separation rule; the issue is a specific pharmacologic interaction at the blood-brain barrier.
Option D: Option D is incorrect: the danger is greater brain penetration via reduced P-gp efflux and a compromised barrier, not merely higher plasma levels with the brain spared.
Option E: Option E is incorrect: ivermectin is normally CNS-safe in people with intact P-gp and barrier function; the HIV-related factors are precisely what raise the risk here.

27. [CASE 7 — QUESTION 3] Continuing with the same patient. A trainee asks how ivermectin actually kills the parasites it is being used against. What is ivermectin's antiparasitic mechanism of action?

A) It inhibits acetylcholinesterase, causing acetylcholine to accumulate
B) It prolongs the open state of voltage-gated sodium channels
C) It blocks GABA-A receptors, causing excitation
D) It physically suffocates the parasite by blocking its breathing pores
E) It binds and opens glutamate-gated chloride channels (an ion channel found in invertebrate nerve and muscle cells), causing chloride influx, hyperpolarization, paralysis, and death of the parasite

ANSWER: E

Rationale:

Ivermectin binds to and opens glutamate-gated chloride channels, which are present in invertebrate nerve and muscle cells. The resulting chloride influx hyperpolarizes the cell, producing paralysis and death of the parasite. These channels are essentially confined to invertebrates, and in mammals ivermectin is additionally excluded from the central nervous system by P-glycoprotein, which together account for its selective antiparasitic action and safety.

Option A: Option A is incorrect: acetylcholinesterase inhibition is the malathion mechanism, not ivermectin.
Option B: Option B is incorrect: prolonging sodium-channel opening is the pyrethroid (permethrin) mechanism, not ivermectin.
Option C: Option C is incorrect: GABA-A receptor blockade describes lindane; ivermectin acts on glutamate-gated chloride channels, not by blocking GABA-A receptors.
Option D: Option D is incorrect: physical suffocation through the spiracles is the benzyl alcohol mechanism, not ivermectin.

28. [CASE 7 — QUESTION 4] Continuing with the same patient. His disseminated strongyloidiasis and crusted scabies are both severe and potentially life-threatening, yet the ritonavir interaction and possible barrier compromise raise the risk of ivermectin central nervous system toxicity. What is the most appropriate overall approach?

A) Withhold ivermectin entirely and treat both conditions with topical permethrin alone
B) Give a deliberately subtherapeutic ivermectin dose that cannot possibly cause any toxicity, accepting likely treatment failure
C) Proceed with ivermectin because the life-threatening infections justify its use, while managing the elevated risk through specialist involvement, attention to the interacting drugs and barrier status, and close monitoring, since the benefit in severe hyperinfection outweighs the risk
D) Replace ivermectin with oral metronidazole, which treats both strongyloidiasis and scabies
E) Delay all treatment indefinitely until the HIV is cured

ANSWER: C

Rationale:

In life-threatening strongyloidiasis hyperinfection and severe crusted scabies, ivermectin is often essential, and the benefit outweighs the risk even when factors such as ritonavir co-administration and possible barrier compromise raise the chance of central nervous system toxicity. The correct approach is to proceed under specialist guidance, account for the interacting drugs and barrier status, and monitor closely, rather than withhold a needed drug. This mirrors the principle that ivermectin is justified in severe disease when the threat of the infection exceeds the pharmacologic risk.

Option A: Option A is incorrect: topical permethrin alone treats neither disseminated strongyloidiasis nor crusted scabies adequately, so withholding ivermectin would leave life-threatening infections untreated.
Option B: Option B is incorrect: a deliberately subtherapeutic dose courts treatment failure in a life-threatening infection and is not the rational way to manage the risk.
Option D: Option D is incorrect: metronidazole does not treat strongyloidiasis or scabies, so it is not a substitute for ivermectin here.
Option E: Option E is incorrect: HIV is a chronic condition managed rather than cured, and the severe infections require prompt treatment, so indefinite delay is inappropriate.