**Core Concept:** CD4 count is a measure of the number of CD4+ T-lymphocytes in the blood. These cells play a crucial role in immune response, especially against infections. A low CD4 count can indicate HIV infection, and HIV-infected individuals often present with opportunistic infections and autoimmune disorders.

**Why the Correct Answer is Right:** The correct answer is B - Pneumocystis jiroveci pneumonia (PCP). PCP is a common opportunistic infection in HIV-infected individuals, particularly in those with a CD4 count <200 cells/mm³. Pneumocystis jiroveci is a fungus that infects the lungs, leading to severe respiratory distress. **Why Each Wrong Option is Incorrect:** A. Opportunistic infections are more common in HIV-infected individuals with CD4 counts <200 cells/mm³. However, the correct answer is PCP as it is the most probable cause of respiratory distress in this scenario. C. Autoimmune disorders are associated with a loss of immune tolerance, but the correct answer is PCP, as it is a specific infection rather than a generalized immune deficiency. D. HIV infection can lead to a decrease in the number of CD4 cells, but the correct answer is PCP as it is the specific cause of respiratory distress in this scenario. **Clinical Pearl:** A low CD4 count with respiratory distress should prompt a thorough workup for HIV infection and opportunistic infections like PCP. Earlier diagnosis and initiation of antiretroviral therapy can lead to improved survival and quality of life in HIV patients.