This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
A most important factor associated with progression of HIV positive to AIDS is? 
Select and explain. 
A. Nutrition, B. Viral load, C. CD4 count, D. Age
C: CD4 count
Answer: C: CD4 count 
Explanation: • HIV can infect and kill many different types of cells in the body, but the primary targets are immune cells called CD4 T-cells. The CD4 T-cells are white blood cells that help coordinate the immune system’s response to infection and disease. CD4 T-cells count is the most important factor in determining HIV progression.
• Patients progress to AIDS when their CD4 cell counts drop below 200 cells per microliter of blood. Healthy individuals have a CD4 cell count between 600 and 1,200 cells per microliter of blood. Individuals with a CD4 cell lower than 200 cells per microliter of blood have the greatest risk of developing opportunistic infections.
• The United States Center for Disease Control and Prevention also created a classification system for HIV, based on CD4 count and clinical symptoms, and describes the infection in five groups. In those greater than six years of age it is:
a. Stage 0: the time between a negative or indeterminate HIV test followed less than 180 days by a positive test.
b. Stage 1: CD4 count > 500 cells/µL and no AIDS-defining conditions.
c. Stage 2: CD4 count 200 to 500 cells/µL and no AIDS-defining conditions.
d. Stage 3: CD4 count < 200 cells/µL or AIDS-defining conditions.