**Core Concept**
Visceral leishmaniasis is a severe form of leishmaniasis, a protozoal disease caused by *Leishmania* parasites. The disease is transmitted through the bite of infected female phlebotomine sandflies and affects the internal organs, particularly the spleen, liver, and bone marrow.

**Why the Correct Answer is Right**
The correct answer is *Leishmania donovani*, which is the primary causative agent of visceral leishmaniasis. *L. donovani* infects the reticuloendothelial system, leading to increased susceptibility to infections, weight loss, and hepatosplenomegaly. The parasite multiplies within the macrophages, causing the cells to become filled with amastigotes.

**Why Each Wrong Option is Incorrect**
**Option A:** *Leishmania major* is primarily responsible for cutaneous leishmaniasis, which affects the skin and subcutaneous tissue.
**Option B:** *Leishmania tropica* is another causative agent of cutaneous leishmaniasis, which typically presents with skin lesions.
**Option C:** *Leishmania infantum* is primarily responsible for visceral leishmaniasis in dogs and can also cause the disease in humans, particularly in the Mediterranean region.

**Clinical Pearl / High-Yield Fact**
*Leishmania* parasites can be identified in tissue samples or bone marrow aspirates using Giemsa or Wright's staining. The parasite's presence in these samples is a definitive diagnosis for visceral leishmaniasis.

**Correct Answer:** C. *Leishmania infantum*