In primary angle-closure glaucoma pilocarpine lowers the intraocular pressure by its direct action on the:
**Core Concept**
Primary angle-closure glaucoma is a condition where the pressure inside the eye increases due to the iris blocking the drainage angle of the trabecular meshwork. Pilocarpine, a miotic agent, is used to treat this condition. **Pilocarpine's action on the iris sphincter muscle** is crucial in managing primary angle-closure glaucoma.
**Why the Correct Answer is Right**
Pilocarpine acts by stimulating the muscarinic receptors on the iris sphincter muscle, causing it to contract. This contraction **opens up the drainage angle**, allowing the aqueous humor to drain out of the eye and thus reducing the intraocular pressure. The contraction of the iris sphincter muscle also **pulls the iris away from the trabecular meshwork**, further increasing the outflow of aqueous humor.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because pilocarpine's direct action is not on the ciliary body. While the ciliary body is involved in the production and regulation of aqueous humor, pilocarpine's mechanism of action is more localized to the iris sphincter muscle.
* **Option B:** This option is incorrect because pilocarpine does not act by increasing the production of aqueous humor. In fact, pilocarpine's action is aimed at reducing the intraocular pressure by increasing the outflow of aqueous humor.
* **Option D:** This option is incorrect because pilocarpine's action is not on the blood-aqueous barrier. The blood-aqueous barrier is a structure that separates the aqueous humor from the bloodstream, and pilocarpine's mechanism of action does not involve altering this barrier.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that **pilocarpine is a first-line treatment for primary angle-closure glaucoma**. However, it's crucial to note that the use of pilocarpine should be balanced with the potential for inducing accommodative spasm, which can worsen the condition in some patients.
**Correct Answer:** C.