**Core Concept**
Prophylaxis for *Pneumocystis jirovecii* (formerly *Pneumocystis carinii*) pneumonia is recommended in HIV-positive patients based on CD4+ T-cell count, as the risk of infection increases with declining immune function. The decision is guided by established guidelines from WHO, CDC, and IDSA, which define thresholds for initiating preventive therapy.

**Why the Correct Answer is Right**
The current standard of care, per CDC and IDSA guidelines, recommends *Pneumocystis* prophylaxis in HIV-positive individuals when CD4 count falls below 200 cells/mm³. This threshold reflects a significant drop in immune surveillance, increasing susceptibility to opportunistic infections. At this level, the risk of developing PCP is substantially elevated, and prophylaxis reduces morbidity and mortality. The lower threshold of 100 cells/mm³ is reserved for patients with advanced disease or those already symptomatic.

**Why Each Wrong Option is Incorrect**
Option A: <300 cells/mm³ – This is incorrect because prophylaxis is not routinely recommended at CD4 counts above 200, as the risk of PCP remains relatively low.
Option C: <100 cells/mm³ – This is too restrictive; while PCP risk is high here, prophylaxis is not initiated at this level in all cases due to limited benefit and increased drug toxicity.
Option D: <50 cells/mm³ – This is a level of severe immunosuppression, but prophylaxis is not mandated at this stage due to higher priority for treating other infections and managing advanced disease.

**Clinical Pearl / High-Yield Fact**
Always initiate *Pneumocystis* prophylaxis at CD4 <200 cells/mm³ in HIV patients — this is a key threshold in managing opportunistic infections and preventing early mortality. Do not confuse this with *Toxoplasma* or *Mycobacterium* prophylaxis, which have different thresholds.

✓ Correct Answer: B. < 200 cells / mm³