**Core Concept**
Glaucoma is a group of eye conditions that damage the optic nerve, often due to abnormally high pressure in the eye (intraocular pressure). This increased pressure can lead to progressive vision loss if left untreated.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of glaucoma. Glaucoma is characterized by the damage to the optic nerve due to increased intraocular pressure. This increased pressure is often caused by impaired aqueous humor outflow, which leads to a buildup of fluid in the eye and subsequent damage to the optic nerve. The trabecular meshwork is the primary site of aqueous humor outflow, and its dysfunction can lead to increased intraocular pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** Glaucoma is primarily caused by increased intraocular pressure due to impaired aqueous humor outflow, not by decreased intraocular pressure.
**Option B:** While corticosteroids can increase intraocular pressure and contribute to glaucoma, they are not the primary cause of the condition.
**Option C:** Glaucoma is not primarily caused by increased ciliary body contraction, but rather by impaired aqueous humor outflow.

**Clinical Pearl / High-Yield Fact**
Glaucoma is often asymptomatic in its early stages, making regular eye exams crucial for early detection and treatment. The "optic disc cupping" sign, where the optic disc appears cupped or concave, is a hallmark of glaucoma.

**Correct Answer: C. Increased intraocular pressure is the primary cause of glaucoma.**