Calabar swelling is due to –
**Core Concept**
Calabar swelling, also known as Osler's nodes, is a type of skin lesion associated with certain infectious diseases. It is characterized by painful, indurated, and tender skin swellings, often seen in the lower extremities. This condition is linked to the pathophysiology of the tryptophan-kynurenine pathway.
**Why the Correct Answer is Right**
Calabar swelling is primarily caused by the infection of African trypanosomes, specifically *Trypanosoma brucei rhodesiense* and *Trypanosoma brucei gambiense*. These parasites release a toxin that triggers an immune response, leading to the formation of Calabar swellings. The tryptophan-kynurenine pathway is involved in the production of kynurenine, a metabolite that contributes to the pathogenesis of the disease.
**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect, as Calabar swelling is not caused by the infection of *Leishmania* species, which are responsible for leishmaniasis.
* **Option B:** Incorrect, as Calabar swelling is not associated with the production of antithrombin antibodies, which are seen in heparin-induced thrombocytopenia.
* **Option C:** Incorrect, as Calabar swelling is not a manifestation of the systemic inflammatory response syndrome (SIRS) or sepsis.
**Clinical Pearl / High-Yield Fact**
Calabar swellings are a classic sign of African trypanosomiasis, which can be diagnosed by demonstrating the presence of the parasites in the blood or CSF.
**Correct Answer: D. African trypanosomiasis**