A 68-year-old man comes to the physician because of headache, fatigue, and nonproductive cough for 1 week. He appears pale. Pulmonary examination shows no abnormalities. Laboratory studies show a hemoglobin concentration of 9.5 g/dL and an elevated serum lactate dehydrogenase concentration. A peripheral blood smear shows normal red blood cells that are clumped together. Results of cold agglutinin titer testing show a 4-fold elevation above normal. An x-ray of the chest shows diffuse, patchy infiltrates bilaterally. Treatment is begun with an antibiotic that is also used to promote gut motility. Which of the following is the primary mechanism of action of this drug?

Inhibition of peptide translocation at the 50S ribosomal subunit

Inhibition of bacterial RNA polymerase

Inhibition of folic acid synthesis

Free radical creation within bacterial cells

Inhibition of transpeptidase cross-linking at the cell wall

A 43-year-old man is brought to the emergency department 45 minutes after his wife found him on the floor sweating profusely. On arrival, he is lethargic and unable to provide a history. He vomited multiple times on the way to the hospital. His temperature is 37.3°C (99.1°F), pulse is 55/min, respirations are 22/min, and blood pressure is 98/65 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 80%. Examination shows profuse diaphoresis and excessive salivation. He withdraws his extremities sluggishly to pain. The pupils are constricted and reactive. Scattered expiratory wheezing and rhonchi are heard throughout both lung fields. Cardiac examination shows no abnormalities. There are fine fasciculations in the lower extremities bilaterally. Muscle strength is reduced and deep tendon reflexes are 1+ bilaterally. His clothes are soaked with urine and feces. Which of the following is the mechanism of action of the most appropriate initial pharmacotherapy?

Competitive antagonism of mACh receptors

Non-selective α-adrenergic antagonism

A 26-year-old nurse presents 12 hours after she accidentally stuck herself with a blood-contaminated needle. She reported the accident appropriately and now seeks post-exposure prophylaxis. She does not have any complaints at the moment of presentation. Her vital signs include: blood pressure 125/80 mm Hg, heart rate 71/min, respiratory rate 15/min, and temperature 36.5℃ (97.7℉). Physical examination is unremarkable. The nurse has prescribed a post-exposure prophylaxis regimen which includes tenofovir, emtricitabine, and raltegravir. How will tenofovir change the maximum reaction rate (Vm) and Michaelis constant (Km) of the viral reverse transcriptase?

Vm will decrease, Km will stay the same

Vm will decrease, Km will increase

Vm will stay the same, Km will increase

A 66-year-old woman with chronic obstructive pulmonary disease is brought to the emergency department because of fever, body aches, malaise, and a dry cough. She has smoked one pack of cigarettes daily for 30 years but quit smoking 1 year ago. She lives with her daughter and her granddaughter, who attends daycare. Her temperature is 38.1°C (101°F). Physical examination shows bilateral conjunctivitis, rhinorrhea, and erythematous tonsils without exudates. Further testing confirms infection with an enveloped orthomyxovirus. Administration of a drug with which of the following mechanisms of action is most appropriate?

Inhibition of proton translocation

Inhibition of nucleoside reverse transcriptase

A 49-year-old woman presents to her primary care doctor in late December with malaise. She reports worsening fatigue, myalgias, headache, and malaise that started 1 day ago. She works as a lunch lady at an elementary school. Her past medical history is notable for a distal radius fracture after a fall 2 years ago, but she is otherwise healthy and takes no medications. She does not smoke or drink alcohol. She is married and has 3 adult children who are healthy. Her temperature is 102.9°F (39.4°C), blood pressure is 101/61 mmHg, pulse is 112/min, and respirations are 21/min. On exam, she appears lethargic and uncomfortable but is able to answer questions appropriately. Breath sounds are normal bilaterally. She is started on intravenous fluids and a pharmacologic agent for treatment. Which of the following is the most likely mechanism of action of the drug being used to treat this patient?

Reverse transcriptase inhibitor

RNA-dependent polymerase inhibitor

A 53-year-old woman comes to the physician in February because of a 1-day history of fever, chills, headache, and dry cough. She also reports malaise and generalized muscle aches. She works as a teacher at a local high school, where there was recently an outbreak of influenza. She has a history of intermittent asthma, for which she takes albuterol as needed. She declined the influenza vaccine offered in the fall because her sister told her that a friend developed a flulike illness after receiving the vaccine. She is worried about possibly becoming ill and cannot afford to miss work. Her temperature is 37.9°C (100.3°F), heart rate is 58/min, and her respirations are 12/min. Physical examination is unremarkable. Her hemoglobin concentration is 14.5 g/dL, leukocyte count is 9,400/mm3, and platelet count is 280,000/mm3. In addition to analgesia, which of the following is the most appropriate next step in management?

Live attenuated influenza vaccine

Influenza antivirals for USMLE Step 3: High-Yield Pharmacology Lessons

Neuraminidase Inhibitors - No Escape Artists

Mechanism of Action: Competitively inhibit influenza neuraminidase, an enzyme on the viral envelope. This enzyme normally cleaves sialic acid residues from host cell glycoproteins, allowing the release of newly formed virions.
Effect: By blocking neuraminidase, these drugs cause viral clumping at the host cell surface, preventing the release and spread of progeny virus. They are sialic acid analogs.
Agents & Administration:
- Oseltamivir (Tamiflu): Oral prodrug, widely used.
- Zanamivir (Relenza): Inhaled powder. ⚠️ Can cause bronchospasm; avoid in patients with underlying respiratory disease (asthma, COPD).
- Peramivir (Rapivab): Intravenous (IV), single-dose regimen.
Clinical Window: For treatment, must be administered within 48 hours of symptom onset for maximal efficacy.

📌 NAIs say NAI to viral release!

⭐ Oseltamivir is the most commonly used agent but can be associated with neuropsychiatric side effects (e.g., confusion, delirium), particularly in pediatric populations.

Endonuclease Inhibitor - Cap-Snatching Stopper

Drug: Baloxavir marboxil
Mechanism: Inhibits the cap-dependent endonuclease enzyme of the influenza virus.
- This unique mechanism blocks "cap-snatching," a process where the virus steals the 5' cap from host cell pre-mRNA.
- Effectively halts viral mRNA synthesis and replication.
Administration: Administered as a single oral dose.
📌 Mnemonic: 'BaloXavir boXes the virus in by stopping transcription.'

⭐ Avoid co-administration with polyvalent cation-containing products (e.g., dairy, antacids, mineral supplements) as chelation significantly reduces its absorption.

Adamantanes - The Old Guard

Drugs: Amantadine, Rimantadine
Mechanism: Block the M2 proton channel, which prevents viral uncoating.
- Spectrum: Active against Influenza A only.
Clinical Status: NOT recommended for use.
- Resistance is extremely high (>99% of circulating strains).

Influenza M2 proton channel inhibitor mechanism

📌 Mnemonic: 'A MAN To DINE' (Amantadine/Rimantadine) blocks the M2 door so the virus can't uncoat and dine.

⭐ High-Yield Fact: Amantadine's CNS side effects (ataxia, dizziness, slurred speech) are due to its dopaminergic and NMDA antagonist effects, which are also exploited for its use in Parkinson's disease.

High‑Yield Points - ⚡ Biggest Takeaways

Neuraminidase inhibitors (oseltamivir, zanamivir) block virion release, treating both Influenza A & B.

Adamantanes (amantadine) block the M2 protein, preventing uncoating; only for Influenza A and limited by resistance.

Baloxavir inhibits cap-dependent endonuclease, halting viral mRNA synthesis.

For best results, initiate treatment within 48 hours of symptom onset.

Oseltamivir is oral; inhaled zanamivir carries a risk of bronchospasm.

Amantadine can cause CNS side effects (e.g., ataxia) and livedo reticularis.

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