**Core Concept**
Leishmaniasis is a vector-borne disease caused by parasites of the genus Leishmania, transmitted by the bite of infected female phlebotomine sandflies. There are three main forms of the disease: cutaneous leishmaniasis, mucocutaneous leishmaniasis, and visceral leishmaniasis (kala-azar).

**Why the Correct Answer is Right**
Visceral leishmaniasis, also known as kala-azar, is caused by Leishmania donovani. This parasite infects the reticuloendothelial system, including the spleen, liver, and bone marrow, leading to symptoms such as fever, weight loss, and anemia. The parasite is transmitted through the bite of an infected sandfly, which ingests the parasite while feeding on blood.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a known causative agent of visceral leishmaniasis.
**Option B:** This option is incorrect because it is a causative agent of cutaneous leishmaniasis, not visceral leishmaniasis.
**Option C:** This option is incorrect because it is a causative agent of malaria, not visceral leishmaniasis.

**Clinical Pearl / High-Yield Fact**
It is essential to note that visceral leishmaniasis can be fatal if left untreated, and diagnosis often relies on clinical suspicion, serological tests, and histopathological examination of spleen or bone marrow biopsy.

**Correct Answer:** D. Leishmania donovani