**Core Concept**
Pilocarpine, an anticholinergic medication, lowers intraocular pressure (IOP) in primary open-angle glaucoma by increasing the outflow of aqueous humor through the uveoscleral pathway. This action is mediated by its effect on the ciliary muscle and the trabecular meshwork.

**Why the Correct Answer is Right**
Pilocarpine acts on the muscarinic receptors in the ciliary muscle, causing contraction and relaxation of the trabecular meshwork. This increases the outflow of aqueous humor through the uveoscleral pathway, thereby decreasing IOP. The increased outflow is due to the relaxation of the scleral spur and the opening of the trabecular meshwork.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because pilocarpine does not lower IOP by increasing the production of aqueous humor, but rather by increasing its outflow.
* **Option B:** This option is incorrect because pilocarpine does not act on the iris sphincter muscle to lower IOP.
* **Option C:** This option is incorrect because pilocarpine does not lower IOP by decreasing the production of aqueous humor.

**Clinical Pearl / High-Yield Fact**
Pilocarpine is a muscarinic agonist that increases the outflow of aqueous humor through the uveoscleral pathway, making it an effective treatment for primary open-angle glaucoma. Its use requires careful monitoring of IOP and patient response, as it can cause pupil constriction and accommodation spasm.

**Correct Answer: C. The ciliary muscle**