**Core Concept**
River blindness, also known as onchocerciasis, is a parasitic disease caused by the filarial worm **Oncocerca volvulus**. This disease is transmitted through the bite of an infected blackfly, leading to severe eye and skin lesions.

**Why the Correct Answer is Right**
The correct answer is **B. Oncocerca volvulus** because this filarial worm is the causative agent of onchocerciasis. The worm's larvae, known as microfilariae, are released into the bloodstream and eventually migrate to the skin and eyes, causing inflammation and damage. **Oncocerca volvulus** is found in the subcutaneous tissue, particularly in the skin and around the eyes, where it causes the characteristic symptoms of river blindness.

**Why Each Wrong Option is Incorrect**
**Option A:** Drinking river water without boiling is a common mode of transmission for other diseases, such as Guinea worm disease, but it is not the primary cause of river blindness.

**Option C:** Toxoplasma canis is not related to river blindness; it is a different type of parasite that causes toxoplasmosis, an infection that can affect the eyes, but not in the same way as onchocerciasis.

**Option D:** Glaucoma is a group of eye conditions that damage the optic nerve, often due to increased pressure in the eye. While glaucoma can cause blindness, it is not related to river blindness.

**Clinical Pearl / High-Yield Fact**
River blindness is a major public health concern in sub-Saharan Africa, where it is estimated to affect over 30 million people. Early detection and treatment with ivermectin can help prevent the progression of the disease and reduce the risk of blindness.

**✓ Correct Answer: B. Oncocerca volvulus**