**Core Concept**
Prolonged Undifferentiated Fever (PUO) is a clinical syndrome characterized by fever persisting for more than three weeks without a clear diagnosis. Viral causes play a significant role in PUO, often presenting with non-specific symptoms.

**Why the Correct Answer is Right**
Leptospirosis is a zoonotic infection caused by the bacterium *Leptospira*, not a virus. It typically presents with symptoms such as fever, headache, and muscle aches, often accompanied by jaundice and renal failure in severe cases. Leptospirosis is usually contracted through contact with contaminated water or soil, and its diagnosis relies on serological tests or PCR.

**Why Each Wrong Option is Incorrect**
**Option A:** Epstein-Barr Virus (EBV) is a common cause of PUO, particularly in immunocompetent individuals. EBV infection can manifest as infectious mononucleosis, which often presents with fever, sore throat, and lymphadenopathy.
**Option B:** Cytomegalovirus (CMV) is another viral cause of PUO, especially in immunocompromised patients. CMV infection can lead to a range of symptoms, including fever, fatigue, and organomegaly.
**Option C:** Human Immunodeficiency Virus (HIV) can also cause PUO due to its chronic infection and subsequent immune suppression. HIV-infected individuals often present with non-specific symptoms, including fever, weight loss, and fatigue.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with PUO, it is essential to consider the possibility of an underlying viral infection, particularly in immunocompromised individuals. A thorough medical history, physical examination, and laboratory tests, including serological and molecular diagnostics, can aid in identifying the causative agent.

**✓ Correct Answer: D. Leptospirosis**