**Core Concept**
Calabar swelling, also known as loiasis, is a type of skin reaction that occurs in response to the migration of Loa loa microfilariae through the skin. This phenomenon is a classic clinical manifestation of loiasis, a filarial infection caused by the parasitic worm Loa loa.

**Why the Correct Answer is Right**
Loa loa microfilariae are known to cause Calabar swelling due to their unique migratory behavior. When Loa loa microfilariae migrate through the skin, they can cause localized inflammation, leading to the characteristic swelling. This reaction is thought to be an immune response to the presence of the microfilariae. The enzyme responsible for the pathogenesis of loiasis is not clearly defined, but it is believed to be related to the host's immune response to the microfilariae.

**Why Each Wrong Option is Incorrect**
**Option A:** Onchocerca volvulus is the causative agent of onchocerciasis, also known as river blindness, which is characterized by skin and eye lesions, but not Calabar swelling. Onchocerca volvulus microfilariae are known to cause intense itching and skin lesions, but they do not cause the characteristic Calabar swelling seen in loiasis.

**Option C:** Brugia malayi is another filarial parasite that causes lymphatic filariasis, characterized by lymphedema and elephantiasis. It does not cause Calabar swelling.

**Option D:** Wuchereria bancrofti is also a causative agent of lymphatic filariasis, similar to Brugia malayi. It does not cause Calabar swelling.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Calabar swelling is a characteristic feature of loiasis, and its presence can aid in the diagnosis of Loa loa infection.

**✓ Correct Answer: B. Loa loa**