**Core Concept:** Kala azar is a severe form of leishmaniasis, a group of diseases caused by the Leishmania parasite. It is transmitted through the bite of an infected sandfly.

**Why the Correct Answer is Right:** Kala azar, also known as visceral leishmaniasis, is primarily caused by Leishmania donovani. The parasite is transmitted to humans through the bite of an infected sandfly, particularly Phlebotomus argentipes, which is prevalent in the Indian subcontinent, hence the term "Kala" in Kala azar.

**Why Each Wrong Option is Incorrect:**
A. Leishmaniasis is caused by Leishmania parasites, but the specific type responsible for Kala azar is L. donovani. Leishmania infantum is responsible for cutaneous leishmaniasis (CL), and Leishmania mexicana causes mucocutaneous leishmaniasis.

B. Transmission through mosquitoes, as seen in malaria, is incorrect for Kala azar.

C. The correct answer is not "sandflies" but "sandflies" is a common misconception. The correct vector is Phlebotomus argentipes.

D. The correct answer is not "typhus fever," which is caused by Rickettsia, but this option provides a misconception about the vector.

**Clinical Pearl:** The misconception of "sandflies" in the correct answer highlights the importance of understanding the correct vector and disease entity while answering MCQs.

**Correct Answer:** Kala azar is transmitted through Phlebotomus argentipes.