**Core Concept**
Kala-azar, also known as visceral leishmaniasis, is a protozoan disease caused by Leishmania donovani, transmitted through the bite of infected female phlebotomine sandflies. The disease primarily affects the reticuloendothelial system, leading to symptoms such as fever, weight loss, and organ enlargement.

**Why the Correct Answer is Right**
The correct answer is B. Kala-azar because Leishmania donovani, the causative agent of kala-azar, is transmitted by the bite of phlebotomine sandflies, specifically Phlebotomus argentipes in the Indian subcontinent. The sandfly ingests the parasite while feeding on the blood of an infected mammal, and then transmits it to a new host through its bite. This vector-borne transmission is a critical aspect of kala-azar epidemiology.

**Why Each Wrong Option is Incorrect**
**Option A:** Epidemic typhus is transmitted by the human body louse (Pediculus humanus corporis), not phlebotomine sandflies. It is caused by Rickettsia prowazekii.
**Option C:** Relapsing fever is transmitted by the bite of infected soft ticks (genus Ornithodoros) or lice (genus Pediculus), not phlebotomine sandflies. It is caused by Borrelia species.
**Option D:** Trench fever is transmitted by the bite of infected body lice (Pediculus humanus corporis), not phlebotomine sandflies. It is caused by Bartonella quintana.

**Clinical Pearl / High-Yield Fact**
In endemic areas, kala-azar can be diagnosed using the spleen size criterion, where a spleen size > 2 cm below the costal margin in children or > 5 cm in adults is highly suggestive of the disease. This simple clinical criterion can aid in early diagnosis and prompt treatment.

**✓ Correct Answer: B. Kala-azar**