**Core Concept**
River blindness, also known as onchocerciasis, is a parasitic disease caused by infection with the filarial nematode Onchocerca volvulus. This disease is transmitted to humans through the bite of an infected blackfly, primarily in sub-Saharan Africa.

**Why the Correct Answer is Right**
Onchocerca volvulus is a microfilarial parasite that lodges in the skin and subcutaneous tissues of its host, leading to severe skin and eye lesions. The parasite's microfilariae cause inflammation and damage to the cornea, resulting in blindness. The disease is characterized by intense itching, nodules under the skin, and vision loss due to corneal opacification. The pathogenesis of onchocerciasis involves the immune response to the parasite, which leads to the release of inflammatory cytokines and the activation of immune cells.

**Why Each Wrong Option is Incorrect**
**Option B:** Loa Loa is the causative agent of loiasis, a different filarial disease characterized by "African eye worm" due to the visible migration of the adult worm through the conjunctiva. Loa Loa does not cause river blindness.
**Option C:** Ascaris lumbricoides is a large intestinal nematode that causes ascariasis, a disease characterized by gastrointestinal symptoms and potentially severe complications. Ascaris is not associated with river blindness.
**Option D:** Brugia malayi is a filarial nematode that causes lymphatic filariasis, primarily leading to elephantiasis due to lymphatic obstruction. Brugia malayi does not cause river blindness.

**Clinical Pearl / High-Yield Fact**
Onchocerciasis is often referred to as "river blindness" because it is commonly found in communities near fast-flowing rivers and streams in sub-Saharan Africa. This disease highlights the importance of vector control and public health measures in preventing the spread of infectious diseases.

**✓ Correct Answer: A. Onchocerca**