This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infection, Subject name: Medicine
prophylaxis for Pneumocystis carinii is indicated in HIV positive patents When CD4 count is- 
Select and explain. 
A. <300 cells/mm3, B. < 200 cells / mm3, C. < 100 cells/mm3, D. < 50 cells / mm3
B: < 200 cells / mm3
Answer: B: < 200 cells / mm3 
Explanation: Low CD4 count correlated with risk of PCP (p < 0.0001); 79% had CD4 counts less than 100/microl and 95% had CD4 counts less than 200/microl. Bacterial pneumonia has been linked to several HIV-related factors, including CD4 counts, having an uncontrolled viral load, and not being on antiretroviral therapy. HIV-positive people with a weak immune system, paicularly a CD4 count below 200 cells/mm3, are at greatest risk. Ref Harrison20th edition pg 1107