**Question:** A child presented with one sided leg and scrotal swelling with hydrocoele and ocular lesions. Which parasite is responsible: March 2013

**Core Concept:** Hydrocoele is a condition characterized by the accumulation of fluid in the tunica vaginalis, a pouch of peritoneum surrounding the testis. Ocular lesions in this context could be indicative of a disseminated infection.

**Correct Answer: D. Onchocerca volvulus (Irritable worm syndrome)**

**Why the Correct Answer is Right:** Onchocerca volvulus is a parasitic worm (filarial) transmitted by the bite of infected blackflies. When the adult worms die, they release larvae (microfilariae) that can invade surrounding tissues, leading to the symptoms observed in this case. The irritable worm syndrome is a manifestation of severe onchocerciasis, where the microfilariae migrate to the skin and subcutaneous tissue causing an inflammatory reaction. The hydrocoele and ocular lesions are part of this syndrome.

**Why Each Wrong Option is Incorrect:**

A. Filariasis is a group of diseases caused by filarial worms, including Onchocerca volvulus. However, the other options do not specifically mention O. volvulus as the causative agent.
B. Onchocerciasis is the infection caused by O. volvulus, but the correct answer is Onchocerca volvulus itself, not Onchocerciasis.
C. Onchocerciasis is another term for Onchocerca volvulus infection, but the correct answer is O. volvulus, not Onchocerciasis.
E. Onchocerciasis is a broader term for O. volvulus infection, but the correct answer is O. volvulus itself, not Onchocerciasis.

**Clinical Pearl:** Onchocerciasis can cause severe skin and ocular manifestations, which should prompt a thorough history and examination to identify the transmission vector (blackfly bites) and environmental context. Early diagnosis and treatment can prevent significant morbidity and mortality.