**Question:** Progressive multifocal leukoencephalopathy is due to

A. JC virus infection
B. Herpes simplex virus infection
C. Human immunodeficiency virus (HIV) infection
D. Varicella-zoster virus infection

**Correct Answer:** C. Human immunodeficiency virus (HIV) infection

**Core Concept:** Progressive multifocal leukoencephalopathy (PML) is a rare demyelinating disease of the central nervous system (CNS) caused by the reactivation of the JC virus in individuals with a compromised immune system. PML is primarily associated with HIV infection, which weakens the immune system and allows the JC virus to infect the white matter of the brain.

**Why the Correct Answer is Right:** PML is primarily caused by the human immunodeficiency virus (HIV) infection, leading to a compromised immune system. In its case, the immune system is not strong enough to fight off the JC virus, which is responsible for the disease. The other options are less common causes of PML:

A. JC virus infection: While JC virus is the causative agent, HIV weakens the immune system allowing the virus to cause PML.
B. Herpes simplex virus infection: Herpes simplex virus is not the cause of PML. HIV is.
D. Varicella-zoster virus infection: Varicella-zoster virus is responsible for chickenpox and shingles, not PML.

**Why Each Wrong Option is Incorrect:**

A. Herpes simplex virus infection: PML is primarily caused by the JC virus, not herpes simplex virus. The weakening of the immune system by HIV allows the JC virus to cause PML.
B. Varicella-zoster virus infection: Varicella-zoster virus causes chickenpox and shingles, not PML. HIV weakens the immune system, allowing the JC virus to cause PML.
C. HIV infection: Although HIV is the primary cause of PML due to its effect on the immune system, the correct answer is C, as HIV is the cause of the compromised immune system, not PML itself.
D. Varicella-zoster virus infection: As mentioned earlier, this virus causes chickenpox and shingles, not PML. HIV weakens the immune system, allowing the JC virus to cause PML.

**Clinical Pearl:** PML is a serious neurological complication of HIV infection, mainly affecting immunocompromised individuals. In such cases, the immune system is unable to control the JC virus, leading to the development of PML. HIV weakens the immune system, allowing the JC virus to cause PML.