A 10-year-old boy presents to the emergency department with his parents. The boy complains of fever, neck stiffness, and drowsiness for the last several days. His past medical history is noncontributory. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. There were no sick contacts at home or at school. The family did not travel out of the area recently. His heart rate is 100/min, respiratory rate is 22/min, blood pressure is 105/65 mm Hg, and temperature is 40.5ºC (104.9°F). On physical examination, he appears unwell and confused. His heart rate is elevated with a regular rhythm and his lungs are clear to auscultation bilaterally. During the examination, he experiences a right-sided focal seizure, which is controlled with lorazepam. A head CT reveals bilateral asymmetrical hypodensities of the temporal region. A lumbar puncture is performed and reveals the following: WBC count 25/mm3 Cell predominance lymphocytes Protein elevated The patient is started on a medication to treat the underlying cause of his symptoms. What is the mechanism of action of this medication?

Nucleoside reverse transcriptase inhibition

Binding with ergosterol in the cell membrane

A 2300-g (5-lb 1-oz) male newborn is delivered to a 29-year-old primigravid woman. The mother has HIV and received triple antiretroviral therapy during pregnancy. Her HIV viral load was 678 copies/mL 1 week prior to delivery. Labor was uncomplicated. Apgar scores are 7 and 8 at 1 and 5 minutes respectively. Physical examination of the newborn shows no abnormalities. Which of the following is the most appropriate next step in management of this infant?

Administer zidovudine, lamivudine and nevirapine

Administer lamivudine and nevirapine

A 3-year-old girl is brought to the emergency department because of chest pain for 2 hours. Eight days ago, she was admitted to the hospital for treatment of low-grade fever, malaise, and sore throat. The hospitalization was complicated by pharyngitis with pseudomembrane formation and severe cervical lymphadenopathy briefly requiring intubation. She has not received any routine childhood vaccinations. Serum studies show elevated cardiac troponins. An ECG shows diffuse T wave inversions and prolonged PR interval. Which of the following preventative measures would most likely have prevented this patient's cardiac symptoms?

Denatured bacterial toxin that contains an intact receptor binding site

Antibiotic that binds to penicillin-binding protein 3

Salicylate that inhibits prostaglandin synthesis

Electrolyte that reduces cardiomyocyte excitability

Immunoglobulin that targets circulating proteins

A 6-month-old male presents for a routine visit to his pediatrician. Two months ago, the patient was seen for tachypnea and wheezing, and diagnosed with severe respiratory syncytial virus (RSV) bronchiolitis. After admission to the hospital and supportive care, the patient recovered and currently is not experiencing any trouble breathing. Regarding the possibility of future reactive airway disease, which of the following statements is most accurate?

“Your child has a greater than 20% chance of developing asthma”

“There is no clear relationship between RSV and the development of asthma.”

“Your child’s risk of asthma is less than the general population.”

“Your child has a less than 5% chance of developing asthma”

“Your child’s risk of asthma is the same as the general population.”

A 33-year-old woman presents to her local clinic in rural eastern India complaining of neck pain and fever. She reports a 4 day history of severe neck pain, neck stiffness, mild diarrhea, and fever. She has not taken her temperature. She works as a laborer and frequently carries heavy weights on her back. She is prescribed a medication and told to come back if her symptoms do not improve. Her symptoms resolve after a couple days. Six months later, she gives birth to a newborn male at 34 weeks gestation. His temperature is 97.8°F (36.6°C), blood pressure is 90/55 mmHg, pulse is 110/min, and respirations are 24/min. On examination, the baby is irritable with a weak cry. Ashen gray cyanosis is noted diffusely. What is the mechanism of action of the drug responsible for this child's presentation?

50S ribosomal subunit inhibitor

DNA-dependent RNA polymerase inhibitor

Dihydropteroate synthase inhibitor

30S ribosomal subunit inhibitor

A 25-year-old man presents with jaw discomfort and the inability to open his mouth fully for about 3 days. About a week ago, he says he cut himself while preparing a chicken dinner but did not seek medical assistance. Five days after the original injury, he started noticing jaw discomfort and an inability to open his mouth completely. He has no history of a serious illness or allergies and takes no medications. The patient says he had received his primary tetanus series in childhood, and that his last booster was more than 10 years ago. His blood pressure is 125/70 mm Hg and temperature is 36.9℃ (98.5°F). On physical examination, the patient is unable to open his jaw wider than 2.5 cm. Head and neck examinations are otherwise unremarkable. There is a 5 cm linear shallow laceration with some granulation tissue on the right index finger without necrosis, erythema, or pus. After wound care and initiation of metronidazole, which of the following is the next best step in the management of this patient?

No further treatment is required

A 26-year-old G1P0 woman is brought to the emergency room by her spouse for persistently erratic behavior. Her spouse reports that she has been sleeping > 1 hour a night, and it sometimes seems like she’s talking to herself. She has maxed out their credit cards on baby clothes. The patient’s spouse reports this has been going on for over a month. Since first seeing a physician, she has been prescribed multiple first and second generation antipsychotics, but the patient’s spouse reports that her behavior has failed to improve. Upon examination, the patient is speaking rapidly and occasionally gets up to pace the room. She reports she is doing “amazing,” and that she is “so excited for the baby to get here because I’m going to be the best mom.” She denies illicit drug use, audiovisual hallucinations, or suicidal ideation. The attending psychiatrist prescribes a class of medication the patient has not yet tried to treat the patient’s psychiatric condition. In terms of this new medication, which of the following is the patient’s newborn most likely at increased risk for?

Right ventricular atrialization

Attention deficit hyperactivity disorder

Respiratory syncytial virus therapies for USMLE Step 3: High-Yield Pharmacology Lessons

RSV Overview - The Tiny Terror

Virus: Enveloped, negative-sense, single-stranded RNA paramyxovirus.
Pathogenesis: Infects respiratory epithelium, causing cell fusion and formation of large multinucleated cells (syncytia). This leads to inflammation, mucus production, and airway obstruction.
Clinical: Leading cause of bronchiolitis and pneumonia in infants <1 year old. Presents with fever, cough, wheezing, and respiratory distress.
Seasonality: Peaks in winter months.

⭐ High-Yield: RSV is the most common cause of lower respiratory tract illness (LRTI) in children worldwide, hospitalizing millions annually.

Monoclonal Antibodies - Protein Bodyguards

*Passive immunity via lab-engineered antibodies to prevent severe RSV disease in high-risk infants. These do not treat active infection.

Palivizumab
- Mechanism: Humanized IgG1 monoclonal antibody that targets a specific epitope (site A) on the RSV fusion (F) protein. This blocks viral entry into host cells.
- Indication: Prophylaxis for infants at high risk:
  - Preterm infants (< 29 weeks gestation)
  - Chronic Lung Disease of Prematurity (CLDP)
  - Hemodynamically significant congenital heart disease.
- Administration: Given as a monthly intramuscular (IM) injection throughout the RSV season (typically November to March).
Nirsevimab
- Mechanism: Next-generation monoclonal antibody, also targets the F protein.
- Advantage: Longer half-life due to a modified Fc region, requiring only a single IM dose for the entire RSV season.

⭐ High-Yield: Palivizumab is a prophylactic antibody, not a vaccine. It provides passive immunity but does not induce an active, long-term memory immune response from the host.

Mechanisms of action of antiviral antibodies

Ribavirin - The Risky Rescue

Mechanism: Guanosine analog; inhibits viral RNA polymerase & IMP dehydrogenase, depleting GTP and causing lethal viral mutations.
Use: Reserved for severe RSV infections in immunocompromised adults (e.g., transplant patients). Aerosolized administration.
Toxicity:
- ⚠️ Extreme Teratogen: Contraindicated in pregnancy (patient or partner).
- Dose-dependent hemolytic anemia.
- Bronchospasm (inhaled route).

⭐ Ribavirin's most tested adverse effect is its teratogenicity, requiring strict contraception for both male and female patients during and for 6 months after therapy.

📌 Mnemonic: RIBA

RNA polymerase inhibitor
IMP Dehydrogenase inhibitor
Bad for Baby
Anemia

Ribavirin chemical structure

High‑Yield Points - ⚡ Biggest Takeaways

Palivizumab is a monoclonal antibody used for prophylaxis in high-risk infants; it is not a treatment for active infection.

It targets the RSV F protein, preventing viral fusion and entry.

Ribavirin is reserved for severe RSV infections, primarily in immunocompromised patients.

A major adverse effect of Ribavirin is teratogenicity, making it contraindicated in pregnancy.

The mechanism of Ribavirin involves inhibiting inosine monophosphate dehydrogenase, depleting GTP.

Supportive care remains the primary management for most RSV cases.

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