A 32-year-old woman from rural Maharashtra presents with CD4 count of 180 cells/μL. Which single feature best distinguishes her risk of opportunistic infection (OI) pattern if she has HIV-1 subtype C versus a hypothetical subtype B infection at the same CD4 threshold?

Explanation

## OI Patterns by HIV Subtype and Geographic Context ### The Epidemiological Paradox: Subtype ≠ Virulence, but Context Matters **Key Point:** At the same CD4 count threshold, the *pattern* of opportunistic infections differs between subtype C and subtype B, driven primarily by **endemic pathogen prevalence** in their respective geographic regions, not by intrinsic subtype virulence. ### Subtype C (India/Africa Context) **High-Yield:** Tuberculosis (TB) is the most common AIDS-defining illness in subtype C–infected individuals, even at higher CD4 counts (>200 cells/μL). - **TB incidence:** 10–100× higher than in subtype B cohorts - **Other OIs:** Bacterial infections (non-typhoidal Salmonella, Streptococcus pneumoniae), cryptococcal meningitis, toxoplasmosis - **Reason:** High TB prevalence in the community; TB reactivation occurs even at CD4 >200 cells/μL - **Vertical transmission:** Higher risk of MTCT; pediatric TB common ### Subtype B (North America/Western Europe Context) - **PCP (Pneumocystis pneumonia):** Most common OI at CD4 <200 cells/μL - **CMV:** Retinitis, esophagitis at CD4 <50 cells/μL - **Cryptococcal meningitis:** Less common than in subtype C regions - **TB:** Rare unless in endemic areas or high-risk groups - **Reason:** Low TB prevalence in the community; PCP prophylaxis standard; different pathogen epidemiology ### OI Comparison Table at CD4 <200 cells/μL | OI | Subtype C (India) | Subtype B (USA/Europe) | Reason | |----|-------------------|------------------------|--------| | **Tuberculosis** | Very common (40–60% of AIDS cases) | Rare (<5%) | Community TB burden | | **PCP** | Uncommon (5–10%) | Most common (30–40%) | Prophylaxis availability, pathogen prevalence | | **CMV** | Uncommon | Common (15–20%) | Prophylaxis, immune reconstitution | | **Cryptococcal meningitis** | Common (5–10%) | Less common (2–5%) | Pathogen prevalence, prophylaxis | | **Bacterial infections** | Common | Less common | Nutritional status, healthcare access | **Mnemonic:** **TB-FIRST** (Subtype C): **T**uberculosis, **B**acterial infections, **F**ungal (cryptococcal), **I**nfections (recurrent), **R**etinitis (CMV less common), **S**almonella, **T**oxoplasmosis. **Clinical Pearl:** In India, a patient with CD4 <200 cells/μL and subtype C should receive **TB preventive therapy (TPT)** and **TB screening** at baseline, regardless of TB symptoms. This is a cornerstone of ART initiation in the Indian context. PCP prophylaxis is less critical than in subtype B cohorts. **Warning:** Do NOT assume subtype C causes faster progression or higher mortality. At equivalent CD4 counts and with access to ART, survival is similar. The difference is in *which* OIs occur, not *how fast* AIDS develops. [cite:Park 26e Ch 8; Harrison 21e Ch 197; WHO Guidelines on TB/HIV Co-infection 2023]