1. A medical student is learning how the herpesvirus family is organized. The family Herpesviridae is divided into three subfamilies based on biological and genomic properties, and the antiviral drug spectra correspond closely to these subfamily boundaries. Which subfamily contains herpes simplex virus type 1 (HSV-1), herpes simplex virus type 2 (HSV-2), and varicella-zoster virus (VZV) — the viruses characterized by rapid replication and latency in sensory ganglia?
A) Betaherpesviruses
B) Alphaherpesviruses
C) Gammaherpesviruses
D) Deltaherpesviruses
E) Poxviruses
ANSWER: B
Rationale:
The alphaherpesviruses comprise HSV-1, HSV-2, and VZV. They are defined by rapid replication, a broad host range, and establishment of latency in sensory ganglia (trigeminal and dorsal root ganglia). This subfamily distinction is clinically important because most licensed antiviral agents, such as acyclovir, target alphaherpesviruses far more potently than the other subfamilies.
Option A: Option A is incorrect: the betaherpesviruses are a separate subfamily that includes cytomegalovirus (CMV), human herpesvirus 6 (HHV-6), and human herpesvirus 7 (HHV-7); these replicate more slowly and establish latency in hematopoietic cells, not sensory ganglia.
Option C: Option C is incorrect: the gammaherpesviruses are Epstein-Barr virus (EBV) and Kaposi sarcoma-associated herpesvirus (KSHV, also called HHV-8); they are lymphotropic and associated with malignant transformation, not the viruses listed.
Option D: Option D is incorrect: there is no "deltaherpesvirus" subfamily in the standard Herpesviridae classification; the family contains only alpha, beta, and gamma subfamilies.
Option E: Option E is incorrect: the poxviruses (e.g., variola, mpox) are an entirely different virus family and are not herpesviruses at all.
2. A resident reviews the subfamily classification of the herpesviruses because it predicts which antiviral agents will be effective. Cytomegalovirus (CMV) replicates more slowly than the herpes simplex viruses, establishes latency in hematopoietic progenitor cells and monocytes, and is the agent of choice target for ganciclovir. To which herpesvirus subfamily does CMV belong?
A) Alphaherpesviruses
B) Gammaherpesviruses
C) Poxviruses
D) Betaherpesviruses
E) Retroviruses
ANSWER: D
Rationale:
CMV is a betaherpesvirus. The betaherpesvirus subfamily also includes human herpesvirus 6 (HHV-6) and human herpesvirus 7 (HHV-7); these viruses replicate more slowly than the alphaherpesviruses and establish latency in hematopoietic cells (CD34-positive progenitors and monocytes for CMV). This classification matters because ganciclovir and valganciclovir are the agents of choice for CMV, whereas acyclovir has minimal activity against it.
Option A: Option A is incorrect: the alphaherpesviruses are HSV-1, HSV-2, and VZV, which replicate rapidly and become latent in sensory ganglia — CMV is not among them.
Option B: Option B is incorrect: the gammaherpesviruses are EBV and KSHV/HHV-8, which are lymphotropic and oncogenic; CMV is not a gammaherpesvirus.
Option C: Option C is incorrect: poxviruses are a distinct, non-herpesvirus family.
Option E: Option E is incorrect: retroviruses (such as HIV) are RNA viruses that reverse-transcribe their genome; CMV is a double-stranded DNA herpesvirus and is not a retrovirus.
3. A student is asked why acyclovir is so selective for virus-infected cells and so well tolerated. Acyclovir is an acyclic nucleoside analogue of guanosine that must be converted to its monophosphate before cellular kinases can complete activation to the triphosphate. Which enzyme performs this first, selectivity-determining phosphorylation step far more efficiently in infected than in uninfected cells?
A) Virus-encoded thymidine kinase
B) Host cell DNA polymerase
C) Viral neuraminidase
D) Host cytochrome P450 (CYP) enzyme
E) Viral protease
ANSWER: A
Rationale:
Acyclovir is selectively phosphorylated to acyclovir monophosphate by virus-encoded thymidine kinase (TK), which is present in HSV-1, HSV-2, and VZV-infected cells. Viral TK converts acyclovir far more efficiently than host kinases convert the unphosphorylated drug, creating a roughly 40- to 100-fold concentration gradient of active drug in infected versus uninfected cells. This is the basis of acyclovir's selectivity and excellent tolerability.
Option B: Option B is incorrect: viral (not host) DNA polymerase is the eventual target of acyclovir triphosphate, but DNA polymerase does not perform the activating phosphorylation; it is inhibited by the already-activated drug.
Option C: Option C is incorrect: neuraminidase is an influenza virus enzyme targeted by drugs such as oseltamivir and has no role in herpesvirus drug activation.
Option D: Option D is incorrect: CYP enzymes metabolize many drugs but do not phosphorylate or activate acyclovir; acyclovir is eliminated largely unchanged by the kidney.
Option E: Option E is incorrect: viral proteases process viral polyproteins (relevant in HIV and hepatitis C therapy) but play no part in acyclovir activation.
4. A resident notes that acyclovir is highly effective for HSV and VZV but has essentially no useful direct activity against CMV. Acyclovir requires a specific viral enzyme for its initial activation. What property of CMV explains its intrinsic resistance to acyclovir?
A) CMV produces a viral protease that destroys acyclovir
B) CMV replicates too rapidly for acyclovir to act
C) CMV lacks a viral thymidine kinase
D) CMV has no DNA polymerase
E) CMV is an RNA virus that acyclovir cannot target
ANSWER: C
Rationale:
CMV lacks a viral thymidine kinase (TK) entirely. Because acyclovir depends on viral TK for the first phosphorylation step that activates it, the absence of this enzyme means acyclovir is never efficiently converted to its active form in CMV-infected cells, making CMV intrinsically resistant. CMV instead encodes a different phosphotransferase (the UL97 gene product) that activates ganciclovir, which is why ganciclovir rather than acyclovir is used for CMV.
Option A: Option A is incorrect: CMV resistance to acyclovir is due to the absence of the activating kinase, not to proteolytic destruction of the drug; herpesvirus proteases do not degrade acyclovir.
Option B: Option B is incorrect: CMV actually replicates more slowly than the alphaherpesviruses, so replication speed does not explain the lack of effect.
Option D: Option D is incorrect: CMV does have a DNA polymerase (the UL54 gene product), which is the target of ganciclovir, foscarnet, and cidofovir; the problem is activation, not the absence of a polymerase.
Option E: Option E is incorrect: CMV is a double-stranded DNA virus, not an RNA virus.
5. A student learns that oral acyclovir has poor and saturable bioavailability (roughly 15% to 30%), which limits the plasma levels achievable by mouth. A related oral agent was developed to overcome this by being rapidly hydrolyzed after absorption to release acyclovir, yielding plasma concentrations three to five times higher than equivalent oral acyclovir. Which drug is this acyclovir prodrug?
A) Ganciclovir
B) Foscarnet
C) Cidofovir
D) Brincidofovir
E) Valacyclovir
ANSWER: E
Rationale:
Valacyclovir is the L-valyl ester prodrug of acyclovir. After oral administration it is rapidly hydrolyzed by intestinal and hepatic valacyclohydrolase to acyclovir, achieving plasma concentrations three to five times higher than equivalent oral acyclovir doses and approximating those reached with intravenous acyclovir. This improved oral exposure is why valacyclovir is favored for outpatient HSV and VZV therapy.
Option A: Option A is incorrect: ganciclovir is a distinct anti-CMV nucleoside analogue, not a prodrug of acyclovir; its own oral prodrug is valganciclovir.
Option B: Option B is incorrect: foscarnet is a pyrophosphate analogue given intravenously and is unrelated structurally to acyclovir.
Option C: Option C is incorrect: cidofovir is an acyclic nucleoside phosphonate, not an acyclovir prodrug.
Option D: Option D is incorrect: brincidofovir is the lipid conjugate prodrug of cidofovir, not of acyclovir.
6. A transplant patient develops CMV disease and the team must select first-line antiviral therapy. Acyclovir is ineffective against CMV. Which agent (and its oral prodrug) is the standard agent of choice for treatment and prophylaxis of CMV?
A) Acyclovir / valacyclovir
B) Ganciclovir / valganciclovir
C) Oseltamivir
D) Cidofovir / brincidofovir
E) Amantadine
ANSWER: B
Rationale:
Ganciclovir and its oral prodrug valganciclovir are the agents of choice for CMV. Ganciclovir is activated in CMV-infected cells by the viral UL97 phosphotransferase and inhibits the CMV DNA polymerase. Valganciclovir achieves systemic exposure equivalent to intravenous ganciclovir and has replaced oral ganciclovir for prophylaxis and maintenance therapy in transplant recipients and HIV patients.
Option A: Option A is incorrect: acyclovir and valacyclovir lack meaningful direct activity against CMV because CMV has no viral thymidine kinase to activate them.
Option C: Option C is incorrect: oseltamivir is a neuraminidase inhibitor for influenza and has no activity against herpesviruses.
Option D: Option D is incorrect: cidofovir and brincidofovir are active against CMV but are reserved for resistant or refractory disease because of toxicity (cidofovir) and limited approved indications (brincidofovir); they are not first-line.
Option E: Option E is incorrect: amantadine is an older anti-influenza A agent and has no role in CMV therapy.
7. A patient on ganciclovir for CMV disease is monitored with twice-weekly complete blood counts. The most common reason for ganciclovir dose reduction or discontinuation relates to a hematologic adverse effect that typically reaches its nadir within the first two weeks of therapy. What is the dose-limiting toxicity of ganciclovir?
A) Severe nephrotoxicity from tubular precipitation
B) Hepatotoxicity with transaminase elevation
C) QT-interval prolongation
D) Myelosuppression, particularly neutropenia
E) Pancreatitis
ANSWER: D
Rationale:
The dose-limiting toxicity of ganciclovir (and valganciclovir) is myelosuppression, specifically neutropenia, which occurs in roughly 15% to 40% of treatment courses and is the most common reason for dose reduction or discontinuation. Neutrophil nadirs typically occur within the first two weeks and are generally reversible; granulocyte colony-stimulating factor (G-CSF) may be used to support the absolute neutrophil count during essential therapy.
Option A: Option A is incorrect: severe nephrotoxicity is characteristic of foscarnet and cidofovir, not ganciclovir; ganciclovir's renal toxicity is comparatively mild though dose adjustment for renal impairment is still required.
Option B: Option B is incorrect: hepatotoxicity is a notable concern with brincidofovir, not the dose-limiting effect of ganciclovir.
Option C: Option C is incorrect: QT prolongation is not a recognized dose-limiting toxicity of ganciclovir.
Option E: Option E is incorrect: pancreatitis is not the characteristic dose-limiting toxicity of ganciclovir; the limiting problem is bone marrow suppression.
8. A student studies foscarnet and notes that, unlike acyclovir and ganciclovir, it does not require any intracellular phosphorylation to become active. Foscarnet inhibits viral DNA polymerase by binding directly to the pyrophosphate-binding site, blocking pyrophosphate release during nucleotide incorporation. Chemically, foscarnet is best described as which kind of compound?
A) A pyrophosphate analogue
B) An acyclic nucleoside analogue of guanosine
C) A neuraminidase inhibitor
D) A monoclonal antibody
E) An acyclic nucleoside phosphonate
ANSWER: A
Rationale:
Foscarnet (phosphonoformic acid) is a pyrophosphate analogue. It inhibits viral DNA polymerase by binding directly to the enzyme's pyrophosphate-binding site, blocking pyrophosphate release during nucleotide incorporation and halting chain elongation. Because it does not require viral kinases for activation, foscarnet is active against thymidine kinase-deficient acyclovir-resistant HSV and against UL97-mutant ganciclovir-resistant CMV.
Option B: Option B is incorrect: that description fits acyclovir, an acyclic nucleoside analogue of guanosine, not foscarnet.
Option C: Option C is incorrect: neuraminidase inhibitors (e.g., oseltamivir) act against influenza and have no relationship to foscarnet's mechanism.
Option D: Option D is incorrect: foscarnet is a small-molecule pyrophosphate analogue, not a monoclonal antibody.
Option E: Option E is incorrect: acyclic nucleoside phosphonate describes cidofovir; although both bypass viral kinases, foscarnet is structurally a pyrophosphate analogue, not a phosphonate nucleoside.
9. A student compares how the herpesvirus antivirals are activated. Acyclovir requires viral thymidine kinase (TK) for its first activation step. Foscarnet and cidofovir do not depend on viral TK at all. What is the key clinical consequence of this difference when an HSV strain has become acyclovir-resistant through a TK mutation?
A) Foscarnet and cidofovir also lose all activity against the strain
B) The strain becomes resistant to every available antiviral
C) Foscarnet and cidofovir retain activity against the acyclovir-resistant strain
D) Acyclovir simply needs to be given at a higher dose to work
E) The strain can only be treated with oseltamivir
ANSWER: C
Rationale:
Because foscarnet and cidofovir do not require viral thymidine kinase for activation, they retain activity against TK-deficient acyclovir-resistant HSV strains. Acyclovir resistance in clinical practice is most often caused by mutations in the viral TK gene that prevent efficient phosphorylation of acyclovir; since foscarnet acts directly on the polymerase and cidofovir is activated by cellular enzymes, both provide a therapeutic escape route, provided the viral DNA polymerase itself remains susceptible.
Option A: Option A is incorrect: this is the opposite of the truth — the TK-independent activation of foscarnet and cidofovir is precisely why they remain effective.
Option B: Option B is incorrect: a TK-mutant strain is not pan-resistant; it remains susceptible to foscarnet and cidofovir.
Option D: Option D is incorrect: raising the acyclovir dose does not overcome loss of TK-mediated activation, because the drug cannot be converted to its active form regardless of dose.
Option E: Option E is incorrect: oseltamivir is an influenza neuraminidase inhibitor with no activity against HSV.
10. A student is asked to connect acyclovir's structure to its mechanism. After activation to acyclovir triphosphate, the drug both competitively inhibits viral DNA polymerase and, once incorporated into the growing viral DNA strand, stops further elongation. Which structural feature of acyclovir explains why incorporation produces obligate chain termination?
A) It contains an extra phosphate that repels incoming nucleotides
B) It binds irreversibly to the viral capsid
C) It carries a bulky aromatic ring that blocks the polymerase active site
D) It chelates magnesium required by the polymerase
E) Its acyclic sugar lacks the 3'-hydroxyl group needed for chain elongation
ANSWER: E
Rationale:
Acyclovir is an acyclic nucleoside analogue whose sugar moiety lacks the 3'-hydroxyl group. DNA chain elongation requires a free 3'-hydroxyl to form the phosphodiester bond with the next incoming nucleotide. Once acyclovir triphosphate is incorporated into the growing viral DNA strand, the absence of this 3'-hydroxyl makes further extension impossible, producing obligate chain termination — in addition to its competitive inhibition of the polymerase relative to deoxyguanosine triphosphate (dGTP).
Option A: Option A is incorrect: chain termination results from the missing 3'-hydroxyl, not from charge repulsion by an extra phosphate.
Option B: Option B is incorrect: acyclovir does not act on the viral capsid; it targets DNA synthesis.
Option C: Option C is incorrect: termination is due to the acyclic sugar lacking the 3'-OH, not to steric blockade by an aromatic ring.
Option D: Option D is incorrect: acyclovir does not work by chelating magnesium; divalent-cation chelation is a feature associated with foscarnet's electrolyte effects, not acyclovir's mechanism.
11. A resident reviews why ganciclovir works against CMV even though CMV has no classical thymidine kinase. Ganciclovir still requires an initial phosphorylation step to become active, which CMV provides through a virus-encoded enzyme. Which CMV gene product performs the first phosphorylation of ganciclovir?
A) The UL54 DNA polymerase
B) The UL97 phosphotransferase
C) Host thymidine kinase
D) The UL56 terminase
E) Viral neuraminidase
ANSWER: B
Rationale:
CMV encodes a viral phosphotransferase, the UL97 gene product, which phosphorylates ganciclovir to its monophosphate form. Cellular kinases then complete the conversion to ganciclovir triphosphate, which competitively inhibits the CMV DNA polymerase and is incorporated into viral DNA. Mutations in UL97 are the most common cause of ganciclovir resistance.
Option A: Option A is incorrect: UL54 is the CMV DNA polymerase — the eventual target of the activated drug — not the enzyme that performs activation.
Option C: Option C is incorrect: host thymidine kinase does not efficiently activate ganciclovir; the selectivity depends on the viral UL97 enzyme.
Option D: Option D is incorrect: UL56 is a subunit of the CMV terminase complex and is the target of letermovir, not the activator of ganciclovir.
Option E: Option E is incorrect: neuraminidase is an influenza enzyme and is unrelated to ganciclovir activation.
12. A patient receiving foscarnet for resistant CMV is monitored with serum electrolytes before every infusion and given mandatory saline prehydration. Foscarnet's major limitation is severe nephrotoxicity, and it also produces a characteristic electrolyte disturbance because the drug chelates divalent cations. Which electrolyte abnormality is most characteristic of foscarnet for this reason?
A) Hypocalcemia
B) Hypernatremia
C) Hyperkalemia
D) Metabolic alkalosis
E) Hyperglycemia
ANSWER: A
Rationale:
Foscarnet chelates divalent cations, and hypocalcemia (from calcium chelation) is the characteristic electrolyte disturbance; hypomagnesemia, hypokalemia, and phosphate abnormalities also occur and require monitoring with each infusion. Foscarnet's dose-limiting problem is severe nephrotoxicity (up to 30% of patients), which is reduced by vigorous saline prehydration before each dose.
Option B: Option B is incorrect: hypernatremia is not the characteristic foscarnet electrolyte effect; the hallmark is hypocalcemia from cation chelation.
Option C: Option C is incorrect: hyperkalemia is not typical — foscarnet is more often associated with hypokalemia.
Option D: Option D is incorrect: foscarnet is not characterized by metabolic alkalosis; its hallmark effects are nephrotoxicity and divalent-cation disturbances.
Option E: Option E is incorrect: hyperglycemia is not a recognized characteristic effect of foscarnet.
13. A clinician preparing to give cidofovir notes the mandatory protocol: oral probenecid before and after each dose plus intravenous saline preloading. Cidofovir's dose-dependent nephrotoxicity arises because the drug is concentrated in proximal tubular cells by a specific transporter. What is the basis for coadministering probenecid?
A) Probenecid neutralizes the acidity of cidofovir in the urine
B) Probenecid increases cidofovir activation to its diphosphate
C) Probenecid prevents hypocalcemia caused by cidofovir
D) Probenecid inhibits the organic anion transporter 1 (OAT1), reducing cidofovir uptake into proximal tubular cells
E) Probenecid accelerates renal clearance of cidofovir
ANSWER: D
Rationale:
Cidofovir is taken up into proximal tubular cells by the organic anion transporter 1 (OAT1), reaching intracellular concentrations that cause tubular injury and progressive renal failure. Probenecid is a competitive OAT1 inhibitor; given orally before and after each cidofovir dose, it reduces cidofovir uptake into the proximal tubule and substantially mitigates nephrotoxicity. Saline preloading is also required, and serum creatinine must be checked before each dose.
Option A: Option A is incorrect: probenecid's benefit comes from blocking transporter-mediated tubular uptake, not from neutralizing urinary acidity.
Option B: Option B is incorrect: probenecid does not enhance the cellular phosphorylation of cidofovir to its active diphosphate.
Option C: Option C is incorrect: hypocalcemia from cation chelation is a foscarnet effect; probenecid is not given with cidofovir to prevent it.
Option E: Option E is incorrect: probenecid actually reduces tubular handling of the drug to protect the kidney; it is not given to speed cidofovir clearance.
14. A transplant patient with controlled CMV is being transitioned from intravenous induction therapy to an oral maintenance agent. The chosen oral drug is an ester prodrug that achieves systemic exposure equivalent to intravenous ganciclovir and has largely replaced oral ganciclovir for prophylaxis and maintenance. Which agent is this?
A) Valacyclovir
B) Foscarnet
C) Valganciclovir
D) Cidofovir
E) Letermovir
ANSWER: C
Rationale:
Valganciclovir is the oral L-valyl ester prodrug of ganciclovir. It is absorbed and rapidly converted to ganciclovir, achieving systemic exposure equivalent to intravenous ganciclovir, and has replaced oral ganciclovir for CMV prophylaxis and maintenance therapy in transplant recipients and HIV patients with controlled disease.
Option A: Option A is incorrect: valacyclovir is the prodrug of acyclovir and is used for HSV and VZV, not for CMV maintenance.
Option B: Option B is incorrect: foscarnet is an intravenous pyrophosphate analogue used for resistant disease, not an oral ganciclovir prodrug.
Option D: Option D is incorrect: cidofovir is a toxic intravenous agent reserved for resistant or refractory infection, not a routine oral maintenance drug.
Option E: Option E is incorrect: although letermovir is oral and used for CMV prophylaxis in stem cell transplant recipients, it is a UL56 terminase inhibitor, not a prodrug of ganciclovir, and is not the IV-ganciclovir-equivalent maintenance agent described here.
15. A patient with CMV disease develops severe, persistent neutropenia on ganciclovir that does not respond adequately to dose reduction. The team wants an effective anti-CMV agent that does not worsen the bone marrow. Which agent is preferred in this situation, and why?
A) Valganciclovir, because it has no overlap in toxicity with ganciclovir
B) Acyclovir, because it is active against CMV without marrow toxicity
C) Cidofovir, because it never causes any organ toxicity
D) Oseltamivir, because it spares the bone marrow
E) Foscarnet, because it lacks myelosuppression
ANSWER: E
Rationale:
Foscarnet is preferred when ganciclovir-related neutropenia is severe because foscarnet does not cause myelosuppression. Its toxicity profile is dominated instead by nephrotoxicity and electrolyte disturbances, so it can maintain anti-CMV activity without further depressing the neutrophil count. It is also active against many ganciclovir-resistant (UL97-mutant) CMV strains.
Option A: Option A is incorrect: valganciclovir is simply the oral prodrug of ganciclovir and shares the same myelosuppressive toxicity, so it would not relieve the neutropenia.
Option B: Option B is incorrect: acyclovir has no meaningful direct activity against CMV because CMV lacks viral thymidine kinase.
Option C: Option C is incorrect: cidofovir does cause significant toxicity — notably dose-dependent nephrotoxicity — so the claim that it never causes organ toxicity is false.
Option D: Option D is incorrect: oseltamivir is an influenza neuraminidase inhibitor with no CMV activity.
16. A student compares cidofovir with brincidofovir. Cidofovir's use is constrained by OAT1-mediated proximal tubular uptake and nephrotoxicity. Brincidofovir (CMX001) was engineered to overcome this. How does brincidofovir reduce the renal toxicity that limits cidofovir?
A) It is given only intravenously to bypass the gut
B) It is a lipid conjugate that enters cells via lipid transport pathways rather than OAT1, reducing proximal tubular exposure
C) It chemically inactivates OAT1 permanently
D) It is co-formulated with probenecid in a single tablet
E) It is renally cleared so rapidly that it never reaches tubular cells
ANSWER: B
Rationale:
Brincidofovir is a lipid (ether-linked) conjugate of cidofovir. Because the lipid conjugate enters cells through lipid transport pathways rather than via the organic anion transporter 1 (OAT1), it largely bypasses the proximal tubular uptake responsible for cidofovir's nephrotoxicity. Intracellular phospholipases then cleave the lipid to release cidofovir, which is phosphorylated to the active diphosphate; the result is improved oral bioavailability and markedly less renal toxicity.
Option A: Option A is incorrect: a defining advantage of brincidofovir is improved oral bioavailability, so it is not simply an intravenous reformulation.
Option C: Option C is incorrect: brincidofovir does not permanently inactivate OAT1; it avoids the transporter by using a different cellular entry route.
Option D: Option D is incorrect: brincidofovir's renal sparing comes from its lipid conjugate design, not from co-formulation with probenecid.
Option E: Option E is incorrect: the mechanism is reduced transporter-mediated uptake into tubular cells, not ultra-rapid renal clearance.
17. Applying concepts established earlier in this set: a patient with advanced HIV has progressive mucocutaneous HSV ulceration that has not improved after five to seven days of adequate intravenous acyclovir, and acyclovir-resistant HSV is strongly suspected. Recall that this resistance is usually due to a thymidine kinase mutation. Which agent is the appropriate next choice?
A) Foscarnet
B) Valacyclovir
C) Higher-dose oral acyclovir
D) Ganciclovir
E) Oseltamivir
ANSWER: A
Rationale:
Foscarnet is the agent of choice for acyclovir-resistant HSV. Acyclovir resistance is usually caused by a viral thymidine kinase (TK) mutation that prevents activation of acyclovir; because foscarnet acts directly on the viral DNA polymerase and requires no viral kinase, it remains effective against TK-deficient strains. Lack of clinical response after five to seven days of adequate intravenous acyclovir is the trigger to switch.
Option B: Option B is incorrect: valacyclovir is simply a prodrug of acyclovir and depends on the same TK activation, so it cannot overcome TK-mediated resistance.
Option C: Option C is incorrect: increasing the acyclovir dose does not help when the virus cannot phosphorylate the drug at all.
Option D: Option D is incorrect: ganciclovir also depends on phosphorylation (by CMV UL97 in CMV) and is not the standard choice for TK-mutant acyclovir-resistant HSV; foscarnet is preferred.
Option E: Option E is incorrect: oseltamivir is an influenza neuraminidase inhibitor with no activity against HSV.
18. Applying earlier concepts to a high-stakes scenario: a neonate presents with skin vesicles and seizures, and there is a maternal history of genital HSV. Neonatal HSV is a medical emergency. Based on the principles already established about acyclovir's safety and the management of serious HSV disease, what is the appropriate immediate antiviral action?
A) Await CSF HSV PCR results before starting any therapy
B) Begin oral valacyclovir suppression only
C) Start foscarnet as first-line therapy
D) Begin empiric high-dose intravenous acyclovir without delay
E) Begin ganciclovir because neonatal HSV requires anti-CMV therapy
ANSWER: D
Rationale:
Neonatal HSV is treated with immediate empiric high-dose intravenous acyclovir (20 mg/kg every 8 hours) without waiting for confirmatory testing, because delay increases mortality and neurological morbidity. Evaluation (CSF HSV PCR, surface swabs, liver function tests) proceeds in parallel, but therapy is not delayed for results. Acyclovir is well tolerated in neonates with adequate hydration.
Option A: Option A is incorrect: therapy must not be delayed for test results in a neonate with suspected HSV; empiric acyclovir is started immediately.
Option B: Option B is incorrect: oral suppression follows completion of the intravenous course; it is not the acute treatment of active disease.
Option C: Option C is incorrect: foscarnet is reserved for acyclovir-resistant disease, not first-line neonatal HSV.
Option E: Option E is incorrect: neonatal HSV is caused by herpes simplex virus, treated with acyclovir; ganciclovir targets CMV and is not the indicated agent.
19. Applying concepts about CMV to transplant risk stratification: in solid organ transplantation, the risk of CMV disease is determined primarily by donor and recipient CMV serostatus. Which donor/recipient serostatus combination carries the highest risk of CMV infection and end-organ disease without prophylaxis?
A) Seronegative donor into seronegative recipient (D-/R-)
B) Seronegative donor into seropositive recipient (D-/R+)
C) Seropositive donor into seronegative recipient (D+/R-)
D) Seropositive donor into seropositive recipient (D+/R+)
E) Serostatus has no effect on CMV risk
ANSWER: C
Rationale:
The highest-risk combination is a CMV-seropositive donor organ transplanted into a CMV-seronegative recipient (D+/R-). This recipient has no pre-existing CMV immunity yet receives latent virus with the graft, carrying a 50% to 80% risk of CMV infection and a 20% to 30% risk of end-organ disease without prophylaxis. D+/R- recipients are also most prone to late-onset CMV disease after prophylaxis ends.
Option A: Option A is incorrect: D-/R- carries the lowest risk because neither donor nor recipient harbors CMV.
Option B: Option B is incorrect: seropositive recipients (R+) face reactivation risk of roughly 10% to 30%, lower than the D+/R- mismatch.
Option D: Option D is incorrect: D+/R+ is also an R+ reactivation-risk scenario, lower than the D+/R- primary-infection scenario.
Option E: Option E is incorrect: serostatus is in fact the primary determinant of CMV risk in transplantation.
20. Applying the activation concepts already established: a transplant patient with CMV disease fails to clear the virus on ganciclovir, and genotypic testing reveals a UL97 mutation. Recall which enzyme UL97 encodes and how foscarnet differs in its activation requirements. What is the expected susceptibility pattern?
A) The virus is now resistant to all antivirals including foscarnet
B) Ganciclovir will work if the dose is doubled
C) Acyclovir is now the best option
D) Only cidofovir-probenecid can possibly help, and foscarnet is useless
E) Ganciclovir is resistant, but foscarnet generally retains activity against the UL97-mutant strain
ANSWER: E
Rationale:
UL97 encodes the CMV phosphotransferase that activates ganciclovir, so a UL97 mutation confers ganciclovir resistance by preventing drug activation. Foscarnet does not require viral phosphorylation — it acts directly on the viral DNA polymerase — so it generally retains activity against UL97-mutant ganciclovir-resistant CMV and is the standard next agent.
Option A: Option A is incorrect: a UL97 mutation alone does not confer foscarnet resistance, so the strain is not pan-resistant; high-level multidrug resistance requires additional UL54 polymerase mutations.
Option B: Option B is incorrect: doubling ganciclovir does not overcome loss of UL97-mediated activation.
Option C: Option C is incorrect: acyclovir has no meaningful activity against CMV regardless of resistance pattern, because CMV lacks viral thymidine kinase.
Option D: Option D is incorrect: foscarnet is not useless — it typically remains active against UL97-mutant CMV and is preferred over cidofovir in this setting.
21. Applying mechanism concepts to a newer agent: a CMV-seropositive hematopoietic stem cell transplant recipient is started on letermovir for CMV prophylaxis. Unlike ganciclovir, foscarnet, and cidofovir — all of which ultimately act on the viral DNA polymerase — letermovir acts on a different viral target. Which statement best characterizes letermovir?
A) It is a DNA polymerase inhibitor that shares cross-resistance with ganciclovir
B) It is a UL56 terminase complex inhibitor with no cross-resistance to ganciclovir, foscarnet, or cidofovir
C) It is a prodrug of ganciclovir
D) It is a neuraminidase inhibitor active against CMV and influenza
E) It causes dose-limiting myelosuppression similar to ganciclovir
ANSWER: B
Rationale:
Letermovir inhibits the CMV terminase complex by targeting its UL56 subunit, a mechanism entirely distinct from the DNA polymerase inhibitors. Because it acts on a different target, it has no cross-resistance with ganciclovir, foscarnet, or cidofovir. Approved in 2017 for CMV prophylaxis in seropositive stem cell transplant recipients, it is notable for lacking myelosuppression and for good oral bioavailability.
Option A: Option A is incorrect: letermovir is not a DNA polymerase inhibitor and does not share polymerase-based cross-resistance with ganciclovir.
Option C: Option C is incorrect: letermovir is a mechanistically novel terminase inhibitor, not a ganciclovir prodrug (that is valganciclovir).
Option D: Option D is incorrect: letermovir is not a neuraminidase inhibitor; neuraminidase inhibitors target influenza, not CMV.
Option E: Option E is incorrect: a key advantage of letermovir is the absence of myelosuppression, in contrast to ganciclovir.
22. Applying safety concepts to a special population: a pregnant woman with primary genital HSV needs antiviral therapy, and the team also notes that some herpesvirus antivirals are avoided in pregnancy. Based on the established safety data, which statement is correct?
A) Acyclovir and valacyclovir are considered safe in pregnancy (Category B), whereas ganciclovir and foscarnet are generally avoided
B) All herpesvirus antivirals are equally contraindicated in pregnancy
C) Ganciclovir is the preferred first-line agent for genital HSV in pregnancy
D) Foscarnet is the safest herpesvirus antiviral in pregnancy
E) No antiviral should ever be used in pregnancy regardless of indication
ANSWER: A
Rationale:
Acyclovir and valacyclovir are classified as Pregnancy Category B and are supported as safe and effective in pregnancy, backed by extensive human experience including the Acyclovir in Pregnancy Registry, which found no increase in birth defects among over 1800 first-trimester exposures. By contrast, ganciclovir and foscarnet are generally avoided in pregnancy given teratogenic potential in animal models and are reserved for maternal life-threatening CMV disease.
Option B: Option B is incorrect: the agents differ substantially — acyclovir and valacyclovir are considered safe, so they are not all equally contraindicated.
Option C: Option C is incorrect: ganciclovir targets CMV and is avoided in pregnancy; acyclovir or valacyclovir is used for genital HSV.
Option D: Option D is incorrect: foscarnet is generally avoided in pregnancy, not the safest agent; acyclovir and valacyclovir carry the favorable safety data.
Option E: Option E is incorrect: antivirals such as acyclovir are appropriately used in pregnancy when indicated — for example, VZV pneumonia carries high maternal mortality if untreated.
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