A 32-year-old man from Delhi presents with a 3-week history of fever, night sweats, and progressive dyspnea. He was diagnosed with HIV infection 6 months ago but has not initiated antiretroviral therapy (ART). On examination, he has oral candidiasis and bilateral crackles on auscultation. Chest X-ray shows bilateral interstitial infiltrates with a ground-glass appearance. CD4+ count is 85 cells/µL. Arterial blood gas shows PaO₂ 65 mmHg on room air. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Pneumocystis jirovecii Pneumonia (PCP) ### Key Clinical Features **Key Point:** PCP is the most common opportunistic infection in HIV patients with CD4+ count <100 cells/µL, presenting with insidious onset of dyspnea, fever, and night sweats over weeks. ### Diagnostic Criteria Met | Feature | Finding | Significance | |---------|---------|---------------| | CD4+ count | 85 cells/µL | PCP risk threshold is <200 cells/µL; this is very high risk | | Presentation | Subacute (3 weeks) | Gradual onset is typical for PCP, not acute bacterial infection | | CXR pattern | Bilateral interstitial infiltrates, ground-glass | Classic PCP appearance; miliary pattern suggests TB | | PaO₂ on room air | 65 mmHg | Significant hypoxemia; PCP causes V/Q mismatch | | Oral candidiasis | Present | Marker of advanced immunosuppression | | ART status | Not initiated | Major risk factor for opportunistic infections | ### Pathophysiology 1. **CD4+ depletion** → loss of cell-mediated immunity 2. **Pneumocystis colonization** → trophozoite proliferation in alveoli 3. **Foamy exudate formation** → impaired gas exchange 4. **Hypoxemia** → disproportionate to radiographic findings (classic PCP paradox) ### High-Yield Diagnostic Approach **High-Yield:** PCP diagnosis is clinical + radiographic; confirmed by: - Induced sputum with silver stain or immunofluorescence (first-line, non-invasive) - Bronchoalveolar lavage (BAL) with staining if sputum negative - LDH elevation (often >400 IU/L, non-specific but supportive) ### Management **Key Point:** First-line treatment is **trimethoprim-sulfamethoxazole (TMP-SMX)** for 21 days. ```mermaid flowchart TD A[PCP suspected<br/>CD4 < 200]:::outcome --> B{Hypoxemia?}:::decision B -->|PaO₂ ≥ 70 mmHg| C[TMP-SMX alone]:::action B -->|PaO₂ < 70 mmHg| D[TMP-SMX + Corticosteroids]:::action D --> E[Prednisone 40 mg BD<br/>x 5 days, then taper]:::action C --> F[Initiate ART<br/>after 2 weeks PCP Rx]:::action D --> F F --> G[PCP prophylaxis<br/>until CD4 > 200]:::action ``` **Clinical Pearl:** Corticosteroids (prednisone) are indicated if PaO₂ <70 mmHg or A-a gradient >35 mmHg; they reduce mortality by ~50% in moderate-to-severe PCP. ### PCP Prophylaxis **Mnemonic: TMP-SMX for CD4 < 200** — Start prophylaxis when CD4+ falls below 200 cells/µL; discontinue when CD4+ >200 cells/µL for >3 months on ART. [cite:Harrison 21e Ch 197]