**Question:** A 50yr old male presented with various subcutaneous nodules over right iliac crest which is around 1cm in diameter. Inside the excised nodules adult worms and microfilariae were detected. Which of the following organism is likely the causative agent?

A. Wuchereria bancrofti
B. Brugia malayi
C. Onchocerca volvulus
D. Dirofilaria immitis

**Correct Answer:** D. Dirofilaria immitis

**Core Concept:** Filariasis is a group of diseases caused by filarial worms, which belong to the phylum Nematoda. These worms are transmitted to humans through the bite of an infected mosquito vector, particularly at night. The symptoms depend on the location and size of the worms.

**Why the Correct Answer is Right:** The correct answer is Dirofilaria immitis because this nematode species is known to cause subcutaneous dirofilariasis, also called heartworm disease. It is transmitted through the bite of infected mosquitoes, causing the worms to reside in the subcutaneous tissues and heart. The presence of microfilariae (immature larvae) and adult worms inside the excised nodules confirms the diagnosis.

**Why Each Wrong Option is Incorrect:**

A. Wuchereria bancrofti: This species causes lymphatic filariasis, with worms lodging in the lymphatic system, not subcutaneous tissues.
B. Brugia malayi: This species causes lymphatic filariasis, also lodging in the lymphatic system, not subcutaneous tissues.
C. Onchocerca volvulus: This species causes onchocerciasis (river blindness), with worms residing in the skin and subcutaneous tissues, not heart and lungs like Dirofilaria immitis.

**Core Concept:** Dirofilariasis is an example of a zoonotic disease, meaning it can be transmitted to humans from animals. In this case, the disease is transmitted from dogs and other mammals to humans, causing subcutaneous dirofilariasis (dog heartworm disease).

**Why Each Wrong Option is Incorrect:**

A. Wuchereria bancrofti: This species causes lymphatic filariasis, which is transmitted through the bite of an infected mosquito and affects the lymphatic system, not subcutaneous tissues.
B. Brugia malayi: This species causes lymphatic filariasis, affecting the lymphatic system, not subcutaneous tissues.
C. Onchocerca volvulus: This species causes onchocerciasis, affecting skin and subcutaneous tissues, not heart and lungs like Dirofilaria immitis.

**Clinical Pearl:** The correct diagnosis requires a high index of suspicion in endemic areas and knowledge of the typical presentation of subcutaneous dirofilariasis. The disease is more common in tropical regions like Southeast Asia and the Caribbean, affecting humans exposed to the mosquito vector. The clinical features include subcutaneous nodules, which can grow and ulcerate, leading to