A 32-year-old man from Delhi presents with fever, cough, and dyspnea for 3 weeks. He reports unprotected sexual contact 6 months ago. On examination, he has oral candidiasis and hepatosplenomegaly. CD4 count is 85 cells/μL. Chest X-ray shows bilateral interstitial infiltrates. Blood cultures and sputum AFB are negative. What is the most likely causative organism of his respiratory infection?

## Clinical Presentation & Diagnosis This patient presents with advanced HIV disease (CD4 <100 cells/μL) and a classic presentation of **Pneumocystis jirovecii pneumonia (PCP)**. ### Key Diagnostic Features **High-Yield:** PCP is the most common opportunistic infection in HIV patients with CD4 <100 cells/μL, particularly in the Indian subcontinent where TB is endemic but PCP remains a leading OI at very low CD4 counts. **Clinical Pearl:** The triad of: - Insidious onset (weeks to months, not acute) - Bilateral interstitial infiltrates on CXR ("ground glass" appearance on HRCT) - Negative sputum AFB and blood cultures (rules out TB and bacterial sepsis) ### Pathophysiology of PCP in HIV 1. CD4 count <100 cells/μL → loss of T-cell immunity 2. *Pneumocystis jirovecii* (formerly *P. carinii*) reactivation or primary infection 3. Trophozoite proliferation in alveolar spaces → foamy exudate 4. Bilateral interstitial pneumonia with hypoxemia ### Diagnostic Confirmation | Test | Finding in PCP | |------|----------------| | **Sputum induction / BAL** | Trophozoites + cysts on Giemsa or silver stain | | **LDH** | Markedly elevated (>400 IU/L) | | **Chest X-ray** | Bilateral interstitial infiltrates (can be normal early) | | **HRCT** | Ground-glass opacities | | **Arterial blood gas** | Hypoxemia, increased A-a gradient | **Key Point:** Sputum AFB negativity excludes TB as the primary diagnosis; the bilateral interstitial pattern and CD4 <100 strongly favor PCP over cryptococcal pneumonia (which typically presents with minimal CXR findings) or CMV (which causes hemorrhagic pneumonitis, not interstitial infiltrates). ### Treatment & Prevention **Mnemonic: TMP-SMX First Line** — Trimethoprim-Sulfamethoxazole is the gold standard for both treatment and prophylaxis. - **Treatment:** TMP-SMX 15–20 mg/kg/day (TMP component) for 21 days - **Prophylaxis:** CD4 <200 cells/μL → TMP-SMX daily (also covers *Toxoplasma* and *Listeria*) - **Adjunctive:** Corticosteroids if PaO~2~ <70 mmHg or A-a gradient >35 mmHg [cite:Harrison 21e Ch 197]