**Core Concept**
Muscarinic agonists mimic the effects of acetylcholine at muscarinic receptors, leading to increased contraction of the iris sphincter muscle and thus decreasing intraocular pressure. This mechanism is particularly useful in the treatment of angle closure glaucoma.

**Why the Correct Answer is Right**
Pilocarpine is a muscarinic agonist that acts on the M3 muscarinic receptor subtype in the iris sphincter muscle, causing it to contract and narrow the pupil. This reduces the angle between the iris and the cornea, which is critical in the treatment of angle closure glaucoma. By reducing the intraocular pressure, pilocarpine helps to alleviate symptoms such as eye pain and blurred vision.

**Why Each Wrong Option is Incorrect**
**Option A:** Atropine is a muscarinic antagonist, not an agonist, and would actually worsen angle closure glaucoma by causing pupil dilation and increasing intraocular pressure.
**Option B:** Ipratropium is also a muscarinic antagonist, primarily used in the treatment of chronic obstructive pulmonary disease (COPD) to prevent bronchospasm.
**Option C:** Bethanechol is a muscarinic agonist but is more commonly used to treat urinary retention in men with benign prostatic hyperplasia (BPH).
**Option D:** Acetylcholine is the endogenous neurotransmitter that binds to muscarinic receptors, but it is not used as a medication in the treatment of angle closure glaucoma due to its short duration of action and other systemic effects.

**Clinical Pearl / High-Yield Fact**
Pilocarpine is a classic example of a drug that acts on a specific receptor subtype (M3 muscarinic receptor) to produce its therapeutic effect. This highlights the importance of understanding receptor pharmacology in clinical practice.

**Correct Answer: C. Pilocarpine**