**Question:** PML is caused by -

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

**Core Concept:** PML stands for Progressive Multifocal Leukoencephalopathy, a rare but serious neurological disease caused by the reactivation of a virus. PML is typically associated with immunocompromised states, where the immune system is unable to effectively control the viral infection.

**Why the Correct Answer is Right:** PML is caused by the JC virus (John Cunningham virus), a member of the polyomavirus family. In immunocompetent individuals, the JC virus remains asymptomatic and latent in the kidneys. However, in immunocompromised individuals, such as those with HIV infection, the virus reactivates and infects the central nervous system (CNS), causing PML.

**Why Each Wrong Option is Incorrect:**

A. Herpes simplex virus (HSV) causes herpes simplex encephalitis, a completely different viral infection of the CNS. HSV does not cause PML.
B. Varicella-zoster virus (VZV) causes chickenpox and shingles, and is not associated with PML.
D. Human immunodeficiency virus (HIV) is associated with PML, but the correct answer is JC virus. HIV can weaken the immune system, leading to PML in some cases, but HIV itself does not cause PML directly.

**Clinical Pearl:** PML is a valuable clinical clue for immunocompromised states, as it serves as a marker for a weakened immune system. This is especially important for differential diagnosis when considering other neurological diseases with similar clinical presentations.

**Correct Answer: A.** JC virus (JCV) causes PML in immunocompromised individuals, highlighting the importance of a strong immune system in preventing this disease.