**Question:** Kala-azar is transmitted by

A. **Anopheles mosquitoes**
B. **Human bite**
C. **Human-to-human**
D. **Direct blood transfusion**

**Core Concept:**
Kala-azar is a severe form of leishmaniasis caused by the protozoan parasite Leishmania donovani. It primarily affects the reticuloendothelial system (spleen, liver, and bone marrow). The disease is transmitted through the bite of infected sandflies, particularly Phlebotomus species.

**Why the Correct Answer is Right:**
Kala-azar is transmitted by Anopheles mosquitoes (Option A) due to the involvement of Phlebotomus species as the vector in the transmission of Leishmania donovani. Sandflies, a type of mosquito, act as vectors for the parasite when they bite an infected individual and then bite a healthy person, injecting the parasite into the new host.

**Why Each Wrong Option is Incorrect:**

B. Human bite (Option B) is incorrect because the disease is transmitted through sandfly bites, not direct human-to-human contact.

C. Human-to-human transmission (Option C) is incorrect because the disease primarily spreads through the bite of infected sandflies, not from person to person directly.

D. Direct blood transfusion (Option D) is incorrect because kala-azar is not transmitted through blood transfusion, but rather through the bite of infected sandflies.

**Clinical Pearl:**
Kala-azar transmission highlights the importance of vector control measures to prevent the disease. In endemic regions, using insecticides to control sandfly populations and implementing proper vector control methods can significantly reduce the incidence of kala-azar cases.