A 36-year-old male with HIV infection (CD4 count 65 cells/mcL) presents with progressive exertional dyspnea, non-productive cough, and low-grade fever. Chest radiography shows bilateral diffuse interstitial infiltrates. Arterial blood gas demonstrates PaO2 62 mmHg on room air. What is the first-line therapeutic antimicrobial regimen for Pneumocystis jirovecii pneumonia (PCP)?

Answer and explanation

Correct answer: D. Trimethoprim-sulfamethoxazole (TMP-SMX) plus adjuvant corticosteroids

  1. Step 1: Confirm opportunistic infection: Severe immunosuppression with bilateral perihilar infiltrates and hypoxemia strongly points to Pneumocystis jirovecii pneumonia (PCP).
  2. Step 2: Select antimicrobial: High-dose intravenous or oral Trimethoprim-sulfamethoxazole (TMP-SMX) is the definitive first-line treatment.
  3. Step 3: Recognize indication for corticosteroids: A room-air or alveolar-arterial gradient mandates adjuvant systemic corticosteroids to prevent inflammatory lung injury.
Answer
Trimethoprim-sulfamethoxazole (TMP-SMX) plus adjuvant corticosteroids.
Why not the others: 'Primaquine with clindamycin' is an alternative for mild-to-moderate disease or sulfa intolerance, not first line for severe PCP; 'azithromycin' treats Mycobacterium avium complex, not PCP; 'vancomycin with piperacillin-tazobactam' covers bacterial pathogens.

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