**Core Concept:** The World Health Organization (WHO) has defined Acquired Immune Deficiency Syndrome (AIDS) based on a set of clinical criteria, known as AIDS case definition. These criteria help identify patients with advanced stages of HIV infection. Major signs in the definition represent severe manifestations of the disease.

**Why the Correct Answer is Right:** In the AIDS case definition, the major signs refer to the most severe manifestations of HIV infection. These include opportunistic infections and malignancies that occur due to the weakened immune system. The correct answer is not included because it does not represent a severe manifestation of the disease.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** **Opportunistic infections** are not a correct answer since they are a result of the weakened immune system and not a major sign of AIDS.

B. **Option B:** **Mental status changes** are not a correct answer because they are not a severe manifestation of HIV infection. Mental status changes are more commonly seen in the later stages of the disease.

C. **Option C:** **Hypoactive sexual desire** is not a correct answer because it is more related to the sexual dysfunction caused by HIV/AIDS rather than a major sign of AIDS.

D. **Option D:** **Hyponatremia** is not a correct answer because it is a laboratory abnormality and not a severe clinical manifestation of HIV infection. Severe hyponatremia could be a complication of AIDS, but it is not a major sign of AIDS itself.

**Clinical Pearl:** Although hyponatremia may be seen in AIDS, it is not a major sign according to the WHO's AIDS case definition. Instead, focus on the severe manifestations like opportunistic infections, malignancies, and organ dysfunction, which are more directly related to the progression of HIV infection.