1. Influenza virus particles are studded with two major surface proteins: hemagglutinin (the protein that lets the virus attach to and enter respiratory cells) and neuraminidase (an enzyme). Late in the replication cycle, newly assembled virus particles bud from the surface of an infected cell but remain stuck to it. What does neuraminidase do that allows these new virus particles to escape and spread to other cells?
A) It triggers the low-pH change in hemagglutinin that drives fusion of the viral envelope with the cell membrane
B) It cleaves capped fragments from host messenger RNA to prime synthesis of viral messenger RNA
C) It cleaves the sialic acid linkages that tether budding virions to the cell surface, releasing them and preventing their aggregation
D) It pumps protons into the virion interior to uncoat the viral genome after entry
E) It inserts into the host cell membrane to form the pore through which the genome enters
ANSWER: C
Rationale:
Neuraminidase is a sialidase: it cleaves the sialic acid residues that otherwise tether newly budded virions to the host cell surface and to one another. Without this cleavage, progeny virions remain bound to the cell and clump together rather than dispersing through the respiratory tract — which is exactly why neuraminidase inhibitors (oseltamivir, zanamivir, peramivir) block viral spread.
Option A: Option A is incorrect: the low-pH conformational change that drives membrane fusion is a function of hemagglutinin, not neuraminidase.
Option B: Option B is incorrect: cleaving capped primers from host pre-messenger RNA (cap-snatching) is performed by the PA subunit of the viral polymerase and is the target of baloxavir, not a neuraminidase function.
Option D: Option D is incorrect: proton influx that uncoats the genome is the function of the M2 ion channel (the adamantane target), not neuraminidase.
Option E: Option E is incorrect: neuraminidase does not form an entry pore; entry and fusion are mediated by hemagglutinin after receptor-mediated endocytosis.
2. A clinician prescribes oral oseltamivir for a patient with early influenza. Oseltamivir is given as an inactive precursor that the body must convert into its active form. Which statement correctly describes how oseltamivir becomes active?
A) It is an ethyl ester prodrug that is hydrolyzed, mainly by a liver enzyme (carboxylesterase), into the active neuraminidase inhibitor oseltamivir carboxylate
B) It is administered already active and requires no metabolic conversion, distributing unchanged to the respiratory tract
C) It must be phosphorylated three times by viral and host kinases before it can inhibit neuraminidase
D) It is activated only after inhalation deposits it directly onto the airway epithelium
E) It is converted to its active form by viral neuraminidase within infected cells
ANSWER: A
Rationale:
Oseltamivir phosphate is an ethyl ester prodrug. After oral absorption it is hydrolyzed — principally by hepatic carboxylesterase 1 — to oseltamivir carboxylate, the active moiety that competitively inhibits neuraminidase. This ester-to-acid activation is why oseltamivir works orally while the active acid itself is poorly absorbed.
Option B: Option B is incorrect: oseltamivir is a prodrug and does require conversion; the agent given fully active and unchanged is zanamivir (inhaled).
Option C: Option C is incorrect: multi-step intracellular phosphorylation describes nucleoside analogue antivirals (for example acyclovir or ribavirin), not the neuraminidase inhibitor oseltamivir.
Option D: Option D is incorrect: oseltamivir is taken orally and activated systemically, not deposited and activated on the airway; inhaled delivery describes zanamivir.
Option E: Option E is incorrect: activation is by host carboxylesterase, not by viral neuraminidase, which is in fact the enzyme the drug inhibits.
3. Baloxavir marboxil is a single-dose oral influenza drug whose mechanism is entirely different from that of the neuraminidase inhibitors. To make its own messenger RNA, influenza steals short capped fragments from the host cell's messenger RNA and uses them as primers — a process called cap-snatching. Which action best describes how baloxavir blocks viral replication?
A) It competitively blocks the neuraminidase active site, preventing release of budding virions
B) It blocks the M2 ion channel, preventing the proton influx needed to uncoat the genome
C) It is incorporated into the growing viral RNA chain and terminates further elongation
D) It binds hemagglutinin and prevents attachment to sialic acid receptors
E) It inhibits the cap-dependent endonuclease activity of the polymerase acidic (PA) subunit, blocking cap-snatching and halting viral messenger RNA synthesis
ANSWER: E
Rationale:
Baloxavir acid (the active form of the prodrug baloxavir marboxil) binds the polymerase acidic (PA) subunit of the influenza RNA-dependent RNA polymerase and inhibits its cap-dependent endonuclease activity. Because cap-snatching is required to prime synthesis of every influenza messenger RNA, blocking this single step halts production of all viral proteins — a mechanism distinct from, and therefore not cross-resistant with, the neuraminidase inhibitors and adamantanes.
Option A: Option A is incorrect: competitive neuraminidase blockade is the mechanism of oseltamivir, zanamivir, and peramivir, not baloxavir.
Option B: Option B is incorrect: M2 ion-channel blockade describes the adamantanes (amantadine, rimantadine).
Option C: Option C is incorrect: chain incorporation and termination describes nucleoside/nucleotide analogues such as remdesivir, not baloxavir, which acts enzymatically on the PA endonuclease.
Option D: Option D is incorrect: baloxavir does not target hemagglutinin-mediated attachment; no licensed influenza drug acts there as its primary mechanism.
4. Amantadine and rimantadine belong to the oldest class of influenza antivirals, the adamantanes. Their antiviral action depends on blocking a specific viral protein that influenza A uses during uncoating. Which protein do the adamantanes block?
A) The neuraminidase enzyme on the virion surface
B) The M2 proton-selective ion channel, preventing the acidification needed to uncoat the viral genome
C) The polymerase acidic (PA) subunit responsible for cap-snatching
D) Hemagglutinin, the protein that mediates attachment and fusion
E) The fusion (F) protein that drives membrane fusion
ANSWER: B
Rationale:
The adamantanes bind within the pore of the M2 ion channel, a proton-selective channel in the influenza A envelope. Blocking M2 prevents the proton influx that acidifies the virion interior, a step required to dissociate the matrix protein from the ribonucleoprotein complex and release the genome into the cytoplasm — so uncoating fails.
Option A: Option A is incorrect: neuraminidase is the target of oseltamivir, zanamivir, and peramivir, not the adamantanes.
Option C: Option C is incorrect: the PA cap-dependent endonuclease is the baloxavir target.
Option D: Option D is incorrect: hemagglutinin mediates attachment and fusion but is not the adamantane target.
Option E: Option E is incorrect: the fusion (F) protein belongs to respiratory syncytial virus and is the target of nirsevimab and palivizumab, not an influenza adamantane target.
5. Zanamivir is a neuraminidase inhibitor that, unlike oseltamivir, is given by oral inhalation using a dry-powder device rather than as a swallowed capsule. What property of zanamivir explains why it is delivered by inhalation?
A) It is a prodrug that can only be activated by enzymes present in the airway lining
B) It is destroyed by stomach acid and therefore cannot survive oral ingestion
C) It selectively binds influenza B neuraminidase, which is concentrated in the upper airway
D) It is a highly polar molecule with negligible oral bioavailability, so inhalation delivers active drug directly to the respiratory tract
E) It must reach the central nervous system, which inhalation achieves more rapidly than oral dosing
ANSWER: D
Rationale:
Zanamivir is highly polar and has negligible oral bioavailability, so swallowing it would not produce useful systemic or airway concentrations. Oral inhalation deposits active drug directly onto the respiratory mucosa where influenza replicates, achieving high local concentration while keeping systemic exposure low.
Option A: Option A is incorrect: zanamivir is administered already active and is not a prodrug requiring airway activation; the prodrug in this class is oseltamivir.
Option B: Option B is incorrect: the limiting factor is poor absorption due to polarity, not acid degradation.
Option C: Option C is incorrect: zanamivir inhibits both influenza A and B neuraminidase, and the inhaled route reflects pharmacokinetics, not B-subtype selectivity.
Option E: Option E is incorrect: zanamivir acts in the respiratory tract, not the central nervous system, and central penetration is not a therapeutic goal.
6. Influenza antivirals work best when started early. Influenza viral replication peaks within roughly 24 to 72 hours of symptom onset and then falls as the immune response clears the infection. Based on this, why is neuraminidase inhibitor treatment most beneficial when begun within 48 hours of symptom onset in an otherwise healthy outpatient?
A) The drug blocks active viral replication, so the greatest benefit occurs while viral replication is still high; once replication has largely ceased there is little left for the drug to suppress
B) The drug must accumulate for two days before reaching therapeutic concentrations in the airway
C) The drug provides lasting immunity only if given before the immune response begins
D) The drug works by reversing lung tissue damage, which is only possible in the first 48 hours
E) The drug eliminates symptoms by directly blocking inflammatory cytokines rather than viral replication
ANSWER: A
Rationale:
Neuraminidase inhibitors act on actively replicating virus by preventing release of progeny virions. Their benefit is therefore time-dependent and tracks the replication curve: starting within 48 hours captures the period of high replication, while later initiation in a healthy outpatient finds replication already declining, leaving less for the drug to suppress.
Option B: Option B is incorrect: oseltamivir reaches therapeutic concentrations promptly; there is no two-day accumulation requirement, and the timing rationale is viral kinetics, not slow drug buildup.
Option C: Option C is incorrect: antivirals do not confer immunity; that is the role of vaccination.
Option D: Option D is incorrect: the drugs suppress replication and do not reverse established tissue damage.
Option E: Option E is incorrect: the mechanism is inhibition of viral replication via neuraminidase blockade, not direct anti-cytokine activity.
7. A learner notes that neuraminidase inhibitors and baloxavir are active against both influenza A and influenza B, but asks why amantadine is described as having no useful activity against influenza B at all. Which statement best explains this difference?
A) Influenza B has no neuraminidase, so any drug targeting surface enzymes is ineffective
B) Influenza B replicates too rapidly for any ion-channel blocker to act in time
C) The adamantanes act on the M2 ion channel, and influenza B has a structurally distinct ion channel (BM2) that the adamantanes do not block
D) Influenza B lacks a polymerase, so it does not require the step amantadine inhibits
E) Influenza B is intrinsically resistant because it carries the S31N mutation from birth
ANSWER: C
Rationale:
Amantadine and rimantadine block the influenza A M2 proton channel. Influenza B does not have M2; it uses a structurally distinct ion channel, BM2, whose pore architecture the adamantanes cannot block — so the adamantanes have no meaningful activity against influenza B regardless of resistance status.
Option A: Option A is incorrect: influenza B does possess neuraminidase (which is why neuraminidase inhibitors work against it); the adamantane issue concerns the ion channel, not neuraminidase.
Option B: Option B is incorrect: replication speed is not the explanation; the target protein is simply absent.
Option D: Option D is incorrect: influenza B does have a functional polymerase; amantadine targets the ion channel, not the polymerase.
Option E: Option E is incorrect: S31N is an acquired resistance mutation in influenza A M2, not a feature of influenza B, which lacks M2 entirely.
8. A hospitalized patient with influenza cannot reliably take oral or inhaled medication. Among the licensed neuraminidase inhibitors, which agent is given intravenously and is therefore suited to this situation?
A) Oseltamivir, given as an oral capsule
B) Zanamivir, given by dry-powder inhalation
C) Baloxavir, given as a single oral tablet
D) Amantadine, given orally
E) Peramivir, given as a single intravenous dose
ANSWER: E
Rationale:
Peramivir is a neuraminidase inhibitor formulated for intravenous administration, typically as a single dose, which makes it the appropriate choice when a patient cannot take enteral or inhaled medication.
Option A: Option A is incorrect: oseltamivir is oral and unsuitable for a patient who cannot take medication enterally.
Option B: Option B is incorrect: zanamivir is delivered by oral inhalation, which requires patient cooperation and adequate airway function and is not an intravenous option (investigational intravenous zanamivir is not the standard licensed route).
Option C: Option C is incorrect: baloxavir is an oral tablet, not an intravenous agent, and is not a neuraminidase inhibitor.
Option D: Option D is incorrect: amantadine is oral and, as an adamantane, is not used for influenza because of universal resistance.
9. Baloxavir is notable for treating influenza with a single oral dose, whereas oseltamivir is taken twice daily for five days. Which pharmacologic property of baloxavir most directly makes single-dose therapy possible?
A) It is eliminated within a few hours, so repeated dosing would be redundant
B) Its active form has a long plasma half-life (approximately 79 hours), so one dose maintains effective concentrations across the treatment window
C) It is stored in lung tissue and slowly released only when viral replication is detected
D) It permanently inactivates the viral polymerase, so the virus can never resume replication after one exposure
E) It stimulates lasting immunity after a single dose, eliminating the need for further drug
ANSWER: B
Rationale:
Baloxavir acid has a long plasma half-life of roughly 79 hours. Because effective drug concentrations persist for days after a single dose, one administration covers the period of active influenza replication — the pharmacokinetic basis for single-dose therapy.
Option A: Option A is incorrect: rapid elimination would argue for more frequent dosing, not single-dose therapy, and is the opposite of baloxavir's actual profile.
Option C: Option C is incorrect: baloxavir is not a lung-depot drug released on demand; its single-dose convenience comes from its long half-life.
Option D: Option D is incorrect: baloxavir reversibly inhibits the cap-dependent endonuclease and does not permanently inactivate the polymerase; resistance substitutions can even arise during treatment.
Option E: Option E is incorrect: antivirals do not generate immunity; durable protection comes from vaccination.
10. The most clinically important neuraminidase inhibitor resistance mutation is H275Y (a histidine-to-tyrosine substitution at position 275 of the N1 neuraminidase). A patient is infected with an influenza strain carrying H275Y. Which consequence does this mutation most characteristically produce?
A) High-level resistance to baloxavir while preserving susceptibility to oseltamivir
B) Loss of all neuraminidase enzymatic activity, rendering the virus noninfectious
C) Resistance to the adamantanes through altered M2 channel geometry
D) High-level oseltamivir resistance with largely preserved susceptibility to zanamivir
E) Equal, complete resistance to oseltamivir, zanamivir, and peramivir
ANSWER: D
Rationale:
H275Y alters how oseltamivir contacts the N1 neuraminidase active site, conferring high-level oseltamivir resistance, while zanamivir — which contacts the site differently — largely retains activity. This is the clinical basis for switching an oseltamivir-resistant patient to zanamivir or intravenous peramivir.
Option A: Option A is incorrect: H275Y is a neuraminidase mutation and does not confer baloxavir resistance (baloxavir targets the PA endonuclease); the directionality is also reversed.
Option B: Option B is incorrect: H275Y alters the drug-binding site without abolishing enzyme activity, so the virus remains viable and transmissible.
Option C: Option C is incorrect: adamantane resistance is conferred by M2 mutations such as S31N, not by the neuraminidase mutation H275Y.
Option E: Option E is incorrect: cross-resistance is partial, not complete; zanamivir susceptibility is characteristically preserved, which is the whole clinical point.
11. Current guidelines do not recommend amantadine or rimantadine for influenza treatment or prophylaxis. Which fact best explains why these once-standard drugs are now considered clinically obsolete for influenza?
A) Essentially all circulating influenza A strains now carry the S31N mutation in M2, which blocks adamantane binding while imposing little fitness cost, so the drugs no longer work
B) The drugs were withdrawn because they caused unacceptable kidney toxicity at standard doses
C) Newer drugs are simply more convenient, but adamantanes remain fully active against current strains
D) The adamantanes were never effective against influenza A and were used only for influenza B
E) Manufacturing of both drugs has ceased, so they are no longer available for any indication
ANSWER: A
Rationale:
Adamantane resistance spread rapidly: the S31N substitution in the M2 channel disrupts drug binding without meaningfully impairing channel function, so it carries little fitness cost and now dominates circulating influenza A (H3N2 and 2009 pandemic H1N1). With near-universal resistance, the drugs are clinically obsolete for influenza.
Option B: Option B is incorrect: the reason is resistance, not a withdrawal for renal toxicity (although dose reduction is needed in renal impairment, that is not why they are obsolete).
Option C: Option C is incorrect: the issue is not mere convenience — current strains are resistant, so the adamantanes are not fully active.
Option D: Option D is incorrect: adamantanes act on influenza A M2 and have no activity against influenza B, the reverse of this claim.
Option E: Option E is incorrect: amantadine remains manufactured and used for non-influenza indications such as Parkinson disease; availability is not the issue.
12. Although amantadine is no longer useful for influenza, it remains a legitimately prescribed drug. For which of the following is amantadine still appropriately used, reflecting its actions in the central nervous system rather than its antiviral effect?
A) Long-term suppression of chronic hepatitis B infection
B) Prophylaxis against respiratory syncytial virus in premature infants
C) Treatment of Parkinson disease and drug-induced extrapyramidal symptoms
D) First-line treatment of influenza B in pregnancy
E) Prevention of cytomegalovirus reactivation after transplantation
ANSWER: C
Rationale:
Amantadine has dopaminergic activity (dopamine agonism and reuptake inhibition) and N-methyl-D-aspartate (NMDA) receptor antagonism in the basal ganglia. These central nervous system actions — independent of any antiviral effect and unaffected by M2 resistance — underlie its retained use in Parkinson disease, drug-induced extrapyramidal symptoms, and related disorders.
Option A: Option A is incorrect: chronic hepatitis B is treated with agents such as nucleoside/nucleotide analogues, not amantadine.
Option B: Option B is incorrect: respiratory syncytial virus prophylaxis in high-risk infants uses monoclonal antibodies (nirsevimab, palivizumab), not amantadine.
Option D: Option D is incorrect: amantadine has no activity against influenza B (which lacks M2), and oseltamivir is preferred in pregnancy.
Option E: Option E is incorrect: cytomegalovirus prevention uses agents such as valganciclovir or letermovir, not amantadine.
13. Respiratory syncytial virus (RSV) prophylaxis in infants relies on monoclonal antibodies such as palivizumab and nirsevimab. Both antibodies prevent infection by binding the same viral surface protein. Which RSV protein do they target, and what does binding accomplish?
A) The attachment glycoprotein (G protein), preventing the virus from being recognized by the immune system
B) The neuraminidase enzyme, preventing release of progeny virions
C) The M2 ion channel, preventing genome uncoating
D) The polymerase acidic subunit, blocking cap-snatching
E) The fusion (F) protein, blocking the conformational change required for the viral envelope to fuse with the host cell membrane
ANSWER: E
Rationale:
Palivizumab and nirsevimab bind the RSV fusion (F) protein. The F protein drives entry by changing shape from a prefusion to a postfusion conformation, fusing the viral envelope with the host cell membrane; antibody binding sterically blocks that conformational change and thereby prevents cell entry. Nirsevimab targets a prefusion-specific epitope and is engineered for an extended half-life, giving season-long protection from a single injection.
Option A: Option A is incorrect: the antibodies target the F protein, not the G attachment glycoprotein, and they neutralize entry rather than merely flagging the virus for immune recognition.
Option B: Option B is incorrect: RSV is not influenza; neuraminidase is not the RSV target.
Option C: Option C is incorrect: the M2 ion channel is an influenza A protein and adamantane target, not an RSV protein.
Option D: Option D is incorrect: the cap-snatching PA subunit is an influenza polymerase target (baloxavir), not the RSV antibody target.
14. Nirmatrelvir-ritonavir (Paxlovid) is an oral COVID-19 treatment in which nirmatrelvir is the antiviral. What is the role of the ritonavir component in this combination?
A) It is the primary antiviral, inhibiting the SARS-CoV-2 main protease alongside nirmatrelvir
B) It is a pharmacokinetic booster that inhibits the liver enzyme CYP3A4, slowing nirmatrelvir breakdown and raising its plasma concentration to effective levels
C) It prevents viral entry by blocking the spike protein from binding its receptor
D) It is a nucleoside analogue that terminates the growing viral RNA chain
E) It neutralizes circulating virus as a monoclonal antibody
ANSWER: B
Rationale:
Ritonavir has little useful antiviral activity at the doses used here; its purpose is pharmacokinetic boosting. By inhibiting CYP3A4 (and P-glycoprotein), ritonavir slows the metabolism of nirmatrelvir, raising and sustaining nirmatrelvir concentrations at therapeutic levels. That same CYP3A4 inhibition is the source of Paxlovid's many clinically important drug interactions.
Option A: Option A is incorrect: nirmatrelvir, not ritonavir, is the main-protease inhibitor; ritonavir is the booster.
Option C: Option C is incorrect: ritonavir does not block spike-receptor binding; no component of this combination acts as an entry inhibitor.
Option D: Option D is incorrect: the chain-terminating nucleoside analogue mechanism describes remdesivir, not ritonavir.
Option E: Option E is incorrect: ritonavir is a small-molecule enzyme inhibitor, not a monoclonal antibody.
15. Nirsevimab has largely replaced palivizumab for routine respiratory syncytial virus (RSV) prophylaxis in infants. Which practical advantage of nirsevimab most accounts for this shift?
A) Nirsevimab is taken as an oral syrup, whereas palivizumab requires injection
B) Nirsevimab provides active long-term immunity, whereas palivizumab does not
C) Nirsevimab treats established RSV infection, whereas palivizumab only prevents it
D) A single intramuscular dose of nirsevimab protects for an entire RSV season, whereas palivizumab requires monthly injections through the season
E) Nirsevimab is active against influenza as well as RSV, broadening its usefulness
ANSWER: D
Rationale:
Nirsevimab is engineered with an extended-half-life Fc region, so one intramuscular dose protects across an entire RSV season; palivizumab must be given monthly (about five doses per season). The marked reduction in injections and the eligibility of all infants under eight months entering their first season are the practical drivers of the shift.
Option A: Option A is incorrect: both are injectable monoclonal antibodies; neither is an oral syrup.
Option B: Option B is incorrect: both provide passive (antibody-mediated) protection, not active long-term immunity.
Option C: Option C is incorrect: these monoclonal antibodies are prophylactic, not treatments for established RSV disease.
Option E: Option E is incorrect: nirsevimab targets the RSV F protein and is not an influenza drug.
16. Remdesivir is an intravenous antiviral used for COVID-19. After intracellular activation to its triphosphate form, how does remdesivir interfere with viral replication?
A) It is an adenosine analogue that is incorporated into the growing viral RNA chain by the RNA-dependent RNA polymerase and then causes delayed termination of chain elongation
B) It competitively blocks the neuraminidase active site to prevent virion release
C) It inhibits CYP3A4 to raise the concentration of a partner antiviral
D) It binds the M2 ion channel to prevent uncoating
E) It blocks cap-dependent endonuclease activity to prevent cap-snatching
ANSWER: A
Rationale:
Remdesivir is a phosphoramidate prodrug of an adenosine analogue. Activated to its triphosphate, it is accepted as a substrate by the viral RNA-dependent RNA polymerase and incorporated into the nascent RNA chain; elongation then stalls a few nucleotides downstream (delayed chain termination), halting genome synthesis.
Option B: Option B is incorrect: neuraminidase blockade describes the influenza neuraminidase inhibitors, not remdesivir.
Option C: Option C is incorrect: CYP3A4 inhibition to boost a partner drug describes ritonavir, not remdesivir.
Option D: Option D is incorrect: M2 ion-channel blockade describes the adamantanes.
Option E: Option E is incorrect: cap-dependent endonuclease inhibition describes baloxavir, an influenza polymerase inhibitor, not remdesivir.
17. An immunocompromised patient on prolonged oseltamivir for influenza A is found to have a strain carrying the H275Y neuraminidase mutation, and the infection is not improving. Using what you know about how this mutation affects different neuraminidase inhibitors, which change in therapy is most rational?
A) Add amantadine, since adamantanes act at a different site and will cover the resistant strain
B) Double the oseltamivir dose, since H275Y only modestly reduces oseltamivir binding
C) Switch to zanamivir or intravenous peramivir, since H275Y characteristically spares susceptibility to these agents
D) Stop all antivirals, since H275Y confers complete resistance to every available drug
E) Switch to palivizumab, since it neutralizes resistant influenza strains
ANSWER: C
Rationale:
H275Y confers high-level oseltamivir resistance but characteristically preserves zanamivir susceptibility because zanamivir contacts the neuraminidase active site differently; intravenous peramivir is another option. Switching to one of these is the rational move, drawing directly on the resistance concept established earlier in this set.
Option A: Option A is incorrect: adamantanes are clinically obsolete for influenza A because of near-universal M2 resistance, so amantadine would not reliably cover the strain.
Option B: Option B is incorrect: H275Y produces high-level oseltamivir resistance, not a modest reduction, so dose escalation is not a reliable solution.
Option D: Option D is incorrect: H275Y does not confer pan-resistance; zanamivir and peramivir typically remain effective, so stopping all antivirals is wrong.
Option E: Option E is incorrect: palivizumab is an anti-RSV monoclonal antibody with no activity against influenza.
18. A patient treated with baloxavir develops prolonged viral shedding, and testing reveals a PA-I38T substitution (an isoleucine-to-threonine change at position 38 of the polymerase acidic subunit). Knowing where baloxavir acts versus where the neuraminidase inhibitors act, what is the expected susceptibility of this strain?
A) It is resistant to all influenza antivirals because PA-I38T alters the shared drug-binding pocket
B) It is resistant to the adamantanes but susceptible to baloxavir
C) It has lost neuraminidase activity and is therefore noninfectious
D) It is resistant to zanamivir but susceptible to oseltamivir
E) It has reduced susceptibility to baloxavir but retains full susceptibility to the neuraminidase inhibitors, which act at a different target
ANSWER: E
Rationale:
PA-I38 substitutions reduce baloxavir binding at the cap-dependent endonuclease, lowering susceptibility to baloxavir. Because the neuraminidase inhibitors act on a completely different protein (the surface neuraminidase), a baloxavir-resistant PA-I38T strain retains full susceptibility to oseltamivir, zanamivir, and peramivir — making them the logical alternative.
Option A: Option A is incorrect: PA-I38T affects only the baloxavir target; there is no shared binding pocket across these drug classes.
Option B: Option B is incorrect: PA-I38T reduces baloxavir susceptibility (it does not preserve it), and adamantane resistance is a separate M2 issue.
Option C: Option C is incorrect: PA-I38T is a polymerase mutation that does not abolish neuraminidase activity or render the virus noninfectious.
Option D: Option D is incorrect: PA-I38T does not selectively impair zanamivir; both neuraminidase inhibitors remain active.
19. The adamantane resistance mutation S31N spread to become near-universal in circulating influenza A, whereas baloxavir PA-I38 resistance, although it can emerge during treatment, has not spread as widely. Drawing on the concept of fitness cost (the degree to which a mutation impairs viral replication in the absence of drug), what best explains this difference?
A) S31N can only arise under drug pressure, whereas PA-I38T arises spontaneously
B) S31N imposes little fitness cost, so resistant virus replicates and transmits nearly as well as wild type, whereas PA-I38 substitutions impose a greater fitness cost that limits their spread
C) PA-I38 substitutions are more transmissible because they enhance viral replication
D) S31N is lethal to the virus and therefore could never have spread in the population
E) Fitness cost has no relationship to whether a resistance mutation spreads
ANSWER: B
Rationale:
A resistance mutation spreads in the population when it confers resistance without substantially impairing replication. S31N disrupts adamantane binding while barely affecting M2 function — a low fitness cost — so it spread to near-universal prevalence. PA-I38 substitutions exact a greater fitness cost, which limits (though does not entirely prevent) their community spread.
Option A: Option A is incorrect: resistant variants are generated by error-prone replication and exist before drug exposure; drug selects rather than creates them, and this is true for both mutations.
Option C: Option C is incorrect: PA-I38 substitutions impose a fitness cost and are not more transmissible than wild type.
Option D: Option D is incorrect: S31N is not lethal — its low fitness cost is precisely why it spread.
Option E: Option E is incorrect: fitness cost is the central determinant of whether resistance spreads, which is the entire concept being applied here.
20. A patient with moderate persistent asthma presents within 24 hours of influenza symptom onset and needs antiviral treatment. Recalling how each neuraminidase inhibitor is delivered, which agent is the most appropriate first choice, and why?
A) Zanamivir, because inhaled delivery places drug directly at the site of infection in an asthmatic
B) Amantadine, because oral dosing avoids any airway effect
C) Baloxavir is contraindicated in all patients with asthma and must be avoided
D) Oseltamivir, because the inhaled route of zanamivir can provoke bronchospasm in patients with underlying airway disease, making an oral agent preferable
E) No antiviral should be given, because all influenza antivirals worsen asthma
ANSWER: D
Rationale:
Inhaled zanamivir can provoke bronchospasm in patients with asthma or chronic obstructive pulmonary disease, so an oral agent is preferred in this population; oseltamivir is the appropriate first choice and is well suited to early treatment.
Option A: Option A is incorrect: the inhaled route is precisely the problem in airway disease, so zanamivir is not the preferred choice here.
Option B: Option B is incorrect: amantadine is clinically obsolete for influenza because of near-universal resistance, regardless of its route.
Option C: Option C is incorrect: baloxavir is not contraindicated across all asthmatic patients; the airway-specific caution applies to inhaled zanamivir.
Option E: Option E is incorrect: withholding antivirals is wrong — early oral treatment is indicated; the concern is limited to the inhaled route.
21. You learned that ritonavir in nirmatrelvir-ritonavir works by inhibiting the liver enzyme CYP3A4. A patient about to start this drug is also taking simvastatin, which is broken down by CYP3A4. Applying the same enzyme-inhibition concept, what is the predicted consequence and appropriate response?
A) Simvastatin levels will rise because its breakdown is blocked, increasing toxicity risk (such as myopathy); the statin should be held during the course
B) Simvastatin levels will fall because ritonavir speeds its metabolism; the statin dose should be increased
C) There will be no interaction, because ritonavir affects only the antiviral nirmatrelvir
D) Nirmatrelvir levels will fall, making the antiviral ineffective; the antiviral should be stopped
E) Simvastatin will inactivate ritonavir, so the antiviral combination should be doubled
ANSWER: A
Rationale:
Ritonavir inhibits CYP3A4, the enzyme that metabolizes simvastatin. Blocking that metabolism raises simvastatin concentrations and the risk of statin toxicity such as myopathy, so simvastatin (and lovastatin) should be held during the short Paxlovid course. This applies the booster's enzyme-inhibition mechanism to a co-prescribed CYP3A4 substrate.
Option B: Option B is incorrect: ritonavir inhibits rather than induces CYP3A4, so simvastatin levels rise, not fall, and increasing the dose would worsen toxicity.
Option C: Option C is incorrect: ritonavir's CYP3A4 inhibition is not confined to nirmatrelvir; it affects any CYP3A4 substrate, which is the basis of Paxlovid's extensive interactions.
Option D: Option D is incorrect: inhibiting CYP3A4 raises nirmatrelvir levels (the intended boosting effect); it does not lower them.
Option E: Option E is incorrect: simvastatin does not inactivate ritonavir, and doubling the antiviral is not an appropriate response.
22. Across this set you have seen that influenza antivirals attack three different steps: virion release (neuraminidase inhibitors), cap-snatching (baloxavir), and genome uncoating (adamantanes). A patient has influenza A confirmed to carry the H275Y neuraminidase mutation but no polymerase resistance mutation. Which agent offers a rational alternative specifically because it acts at a different step than the resistant target?
A) Amantadine, because adamantane resistance and neuraminidase resistance never coexist
B) Oseltamivir at standard dose, because H275Y does not affect it
C) Baloxavir, because it inhibits cap-snatching at the polymerase acidic subunit — a step unaffected by the neuraminidase mutation H275Y
D) Rimantadine, because it is structurally unrelated to the neuraminidase inhibitors
E) Palivizumab, because it neutralizes neuraminidase-resistant influenza
ANSWER: C
Rationale:
H275Y is a neuraminidase mutation. Baloxavir acts at an entirely different step — cap-dependent endonuclease (cap-snatching) on the polymerase acidic subunit — so a neuraminidase mutation does not affect it, making baloxavir a rational alternative (as would zanamivir or peramivir within the neuraminidase class). This applies the three-mechanism framework built across the set.
Option A: Option A is incorrect: adamantanes are clinically obsolete for influenza A because of near-universal M2 resistance, so amantadine is not a reliable choice.
Option B: Option B is incorrect: H275Y confers high-level oseltamivir resistance, so standard-dose oseltamivir is not appropriate.
Option D: Option D is incorrect: rimantadine is an adamantane and shares the universal adamantane resistance; structural unrelatedness to neuraminidase inhibitors does not make it effective.
Option E: Option E is incorrect: palivizumab is an anti-RSV monoclonal antibody with no influenza activity.
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