**Core Concept**
Angle closure glaucoma is a condition characterized by a sudden increase in intraocular pressure (IOP) due to a blockage in the drainage of aqueous humour. This blockage occurs when the iris is pushed forward, obstructing the trabecular meshwork and the canal of Schlemm.

**Why the Correct Answer is Right**
In angle closure glaucoma, the iris is the primary structure responsible for the obstruction of aqueous humour outflow. The iris is a circular, pigmented diaphragm that covers the pupil, and in angle closure glaucoma, it becomes pushed forward, blocking the drainage angle and preventing the aqueous humour from flowing out of the eye. This is often due to a anatomical abnormality, such as a shallow anterior chamber or a thickened iris, which can cause the iris to become stuck in the trabecular meshwork. The resulting blockage leads to a rapid increase in IOP, which can cause damage to the optic nerve and potentially lead to blindness.

**Why Each Wrong Option is Incorrect**
**Option A:** The canal of Schlemm is a structure involved in the drainage of aqueous humour, but it is not the primary site of obstruction in angle closure glaucoma. The aqueous humour flows into the canal of Schlemm, but the blockage occurs at the level of the iris.
**Option B:** The trabecular meshwork is a key structure involved in the drainage of aqueous humour, but it is not the primary site of obstruction in angle closure glaucoma. The trabecular meshwork is actually the site where the aqueous humour flows out of the eye, but in angle closure glaucoma, it is blocked by the iris.
**Option D:** The scleral venous plexus is a network of veins located in the sclera, and it is not directly involved in the drainage of aqueous humour. The aqueous humour is drained through the canal of Schlemm, not the scleral venous plexus.

**Clinical Pearl / High-Yield Fact**
In angle closure glaucoma, the iris is often described as being "pushed forward" or "wedged" in the trabecular meshwork, which can lead to a sudden increase in IOP. This is often due to a anatomical abnormality, such as a shallow anterior chamber or a thickened iris, which can cause the iris to become stuck in the trabecular meshwork.

**✓ Correct Answer: C. Iris. The iris is the primary structure responsible for the obstruction of aqueous humour outflow in angle closure glaucoma.**