A lady from West Rajasthan presented with an ulcer surrounded by erythema on the right leg. Microscopy of the biopsy from the edge of the ulcer showed organisms with dark staining nuclei and kinetoplast. What is the most likely causative agent?

Correct Answer: B. Leishmania tropica

The clinical presentation of an erythematous ulcer on the leg in a patient from West Rajasthan, combined with microscopic findings of organisms with dark-staining nuclei and a kinetoplast, is pathognomonic for cutaneous leishmaniasis caused by Leishmania tropica. The kinetoplast is the hallmark ultrastructural feature of trypanosomatid protozoa—a specialized mitochondrial DNA-containing organelle. In India, L. tropica is endemic in arid and semi-arid regions, particularly Rajasthan, causing the "dry" form of cutaneous leishmaniasis (CL) with well-demarcated, indurated ulcers surrounded by erythema. The organism is an obligate intracellular parasite found within macrophages and histiocytes at the ulcer margin, where it multiplies as amastigotes. The dark-staining nucleus and kinetoplast are visible on routine histology and Giemsa staining. West Rajasthan is a known endemic zone for L. tropica, making this the most epidemiologically likely diagnosis. The "dry" ulcer phenotype (as opposed to the "wet" ulcer of L. donovani visceral disease) and the geographic location are critical discriminators. Indian guidelines (RNTCP/NTEP) recognize L. tropica as the primary cause of CL in Rajasthan and Uttar Pradesh.

Why the other options are wrong

A. Trypanosoma — While Trypanosoma is also a kinetoplastid with a kinetoplast, it causes African sleeping sickness or Chagas disease—not cutaneous ulcers with the described morphology. Trypanosomes are transmitted by tsetse flies (Africa) or reduviid bugs (Americas), not found in Rajasthan. The clinical presentation and geography rule this out entirely. C. Babesia — Babesia is an intraerythrocytic protozoan causing hemolytic anemia and fever, not cutaneous ulcers. It lacks a kinetoplast and is transmitted by Ixodes ticks. The absence of systemic symptoms and the specific ulcer morphology with kinetoplast-bearing organisms exclude Babesia. D. Histoplasma — Histoplasma is a dimorphic fungus causing respiratory or disseminated disease, not primary cutaneous ulcers with erythema. Histoplasma lacks a kinetoplast (it is a fungus, not a protozoan) and would show yeast forms on histology, not the described dark-staining nuclei with kinetoplasts characteristic of Leishmania.

High-Yield Facts

Mnemonics

KINO for Kinetoplast Kinetoplast = Key feature of Leishmania (and Trypanosoma). Intracellular in macrophages. Nucleus dark-staining. Organelle = mitochondrial DNA. Use when you see 'dark nucleus + kinetoplast' → think Leishmania. RAJ for Rajasthan Leishmaniasis Rajasthan = L. tropica (dry CL). Arid zones. Jaundice absent (unlike visceral). Remember: Rajasthan → L. tropica cutaneous; Bihar → L. donovani visceral.

NBE Trap

NBE may pair "kinetoplast" with Trypanosoma to trap students who know both organisms are kinetoplastids but forget the geographic and clinical context. The specific mention of West Rajasthan and the cutaneous ulcer phenotype are the discriminators that exclude Trypanosoma.

Clinical Pearl

In Indian clinical practice, any patient from Rajasthan or Uttar Pradesh presenting with a chronic, indurated, erythematous ulcer that fails to heal should raise suspicion for L. tropica CL. Biopsy with Giemsa staining from the ulcer margin is the gold standard for diagnosis; the presence of amastigotes with a dark nucleus and kinetoplast is diagnostic. Early recognition and treatment with SSG or amphotericin B prevents scarring and disfigurement.

_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology (Protozoa chapter); RNTCP/NTEP guidelines on cutaneous leishmaniasis; Robbins Pathology Ch. 8 (Infectious Diseases)_