What is the definition of AIDS?

The correct choice identifies AIDS (Acquired Immunodeficiency Syndrome) as characterized by a CD4 count below 200 cells/mm³ or the presence of an AIDS-defining illness. This definition is based on established clinical criteria used to diagnose AIDS, which reflects the advanced stage of HIV infection. When the CD4 count falls below this threshold, the immune system is significantly compromised, leading to greater susceptibility to opportunistic infections and certain cancers. These conditions are categorized as AIDS indicator diseases and include conditions such as pneumocystis pneumonia, tuberculosis, and various malignancies.

While other options mention aspects of HIV status or symptoms, they do not meet the formal criteria for diagnosing AIDS. A positive HIV test with symptoms could indicate HIV infection but does not specifically define AIDS, since not every HIV-infected person will have a CD4 count below 200 or an AIDS-related illness. Similarly, a CD4 count below 300 is significant for HIV progression but does not fit the criteria for an AIDS diagnosis. Lastly, a positive HIV test without symptoms indicates an early phase of HIV infection where the individual may still be asymptomatic, and thus does not meet the definition of AIDS.