Which of the following is an AIDS defining illness?

Pneumocystis pneumonia (PCP) is considered an AIDS-defining illness because it is one of the opportunistic infections that significantly indicate a progression to AIDS in individuals infected with the HIV virus. The diagnosis of AIDS is established when a person's CD4 T-cell count falls below a specific threshold (typically below 200 cells/mm³) or when they develop specific opportunistic infections or cancers. PCP is particularly associated with severely compromised immune systems and is often one of the first opportunistic infections seen in those with advanced HIV infection.

In contrast, the other options listed—common cold, influenza, and strep throat—are caused by viruses or bacteria that typically affect individuals with healthy immune systems. These conditions do not serve as indicators of advanced HIV infection or AIDS, as they can occur in anyone regardless of their immunocompromised status. Thus, PCP is uniquely significant as a marker for AIDS due to the severity and implications of the infection in individuals with HIV.