**Core Concept**
Glaucoma is a group of eye conditions that damage the optic nerve, often due to abnormally high pressure in the eye. This pressure can lead to progressive vision loss and, if left untreated, can result in blindness. Glaucoma is a leading cause of irreversible vision impairment worldwide.

**Why the Correct Answer is Right**
Glaucoma is the highest prevalent cause of blindness globally, affecting approximately 80 million people. The increased intraocular pressure in glaucoma damages the optic nerve, leading to visual field defects and potentially complete blindness. The disease often progresses silently, making early detection and treatment crucial to prevent vision loss. The pathophysiology of glaucoma involves the imbalance between the production and drainage of aqueous humor, leading to increased intraocular pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** Corneal opacity, while a significant cause of reversible blindness, is not the highest prevalent cause of blindness worldwide. It often results from infections, injuries, or congenital conditions.
* **Option B:** Diabetic retinopathy is a major cause of blindness in developed countries but ranks lower than glaucoma in terms of global prevalence. It is a complication of diabetes that affects the blood vessels in the retina.
* **Option D:** Trachoma is a leading cause of preventable blindness in developing countries, primarily affecting children. However, its prevalence is lower than that of glaucoma globally.

**Clinical Pearl / High-Yield Fact**
Glaucoma is often asymptomatic in its early stages, with vision loss occurring gradually over years. Regular eye exams are essential for early detection and treatment to prevent irreversible vision impairment.

**✓ Correct Answer: C. Glaucoma**