Which of the following is NOT an example of a direct acting cholinergic agonist?

• A. Carbachol
• B. Pilocarpine
• C. Echothiophate
• D. Bethanechol

Answer: (C)

Direct acting cholinergic agonists stimulate the receptors themselves, mimicking acetylcholine to produce parasympathetic effects such as increased secretions, smooth muscle contraction, and pupil constriction. Bethanechol, carbachol, and pilocarpine are classic examples because they bind to muscarinic (and in some cases nicotinic) receptors directly to activate them. Echothiophate works differently: it’s an organophosphate that inhibits acetylcholinesterase, so acetylcholine accumulates and receptor activity increases indirectly. Because its effect comes from preventing breakdown of acetylcholine rather than directly activating receptors, it is not a direct acting cholinergic agonist.