**Question:** AIDS is a disease of the immune system primarily affecting

A. Lymph nodes
B. Red blood cells
C. Brain
D. Skin

**Core Concept:** Acquired Immunodeficiency Syndrome (AIDS) is a severe stage of Human Immunodeficiency Virus (HIV) infection, which targets and destroys T-helper lymphocytes (CD4+ T cells), leading to a compromised immune system and increased susceptibility to opportunistic infections and cancers.

**Why the Correct Answer is Right:** AIDS is the final stage of HIV infection, where the CD4+ T cell count drops below 200 cells/mm³ or 14% of total lymphocytes. In this stage, the immune system is severely compromised, making the individual highly vulnerable to opportunistic infections and cancers.

**Why Each Wrong Option is Incorrect:**

A. Lymph nodes: AIDS primarily affects CD4+ T cells, not lymph nodes.
B. Red blood cells: AIDS does not primarily affect red blood cells; however, anemia may occur as a result of malnutrition, opportunistic infections, or AIDS-related cancers.
C. Brain: AIDS can cause neurological disorders and infections, but AIDS itself is not primarily associated with brain involvement.
D. Skin: AIDS may cause skin infections or manifestations, but the primary focus of AIDS is the immune system, not skin.

**Clinical Pearl:** HIV infection is transmitted through unprotected sexual contact, contaminated blood, or from mother to child during pregnancy, childbirth, or breastfeeding, leading to a cascade of immunological events culminating in AIDS. Maintaining a comprehensive understanding of HIV infection, CD4+ T cell count, and opportunistic infections is essential for accurate diagnosis and appropriate management of patients with AIDS-related symptoms.