What is the only topical direct cholinergic agonist on the NBEO matrix?

Pilocarpine is the only topical direct cholinergic agonist listed in the choices. This agent acts directly on muscarinic receptors in the eye, which leads to various pharmacological effects, particularly the contraction of the pupillary sphincter muscle and ciliary muscle. This action is significant in the treatment of conditions such as glaucoma, where pilocarpine helps reduce intraocular pressure by facilitating aqueous humor outflow through the trabecular meshwork.

Carbachol, while also a cholinergic agonist, is primarily used in intraocular surgery and not as commonly prescribed as pilocarpine for topical use. Physostigmine is an indirect cholinergic agonist, which means it inhibits the enzyme acetylcholinesterase to increase levels of acetylcholine but does not itself act directly on the receptors. Echothiophate is another indirect cholinergic agent, classified as an organophosphate, and is utilized primarily in specific therapeutic contexts, such as in some cases of accommodative esotropia, rather than as a first-line topical treatment for glaucoma.

Thus, pilocarpine stands out in this list as the classic topical agent directly stimulating cholinergic pathways in ocular pharmacology.