Regarding cholinergic agonists and their clinical uses, all of the following statements are correct EXCEPT:

Explanation

## Analysis of Cholinergic Agonists ### Correct Statements **Option 1 — Pilocarpine (Correct)** - Selective M3 muscarinic agonist - Increases aqueous humour outflow in glaucoma via ciliary muscle contraction - Stimulates salivary and lacrimal glands; used in xerophthalmia - [cite:KD Tripathi 8e Ch 6] **Option 2 — Bethanechol (Correct)** - Selective M3 agonist (resistant to acetylcholinesterase) - Increases gastric motility and acid secretion - Contraindicated in peptic ulcer disease, gastric outlet obstruction - [cite:KD Tripathi 8e Ch 6] **Option 4 — Acetylcholine (Correct)** - Rapidly hydrolyzed by acetylcholinesterase (half-life < 1 minute) - Highly polar; does not cross BBB - Poor oral absorption; no systemic clinical use - [cite:Harrison 21e Ch 432] ### Incorrect Statement — The Answer **Option 3 — Carbachol (INCORRECT)** **Key Point:** Carbachol is a non-selective cholinergic agonist (acts on both nicotinic and muscarinic receptors), but it is **NOT used systemically** for urinary retention. - Carbachol is **highly polar** and does **NOT cross the blood-brain barrier** - It is used **topically** in the eye (glaucoma) and **intramuscularly/subcutaneously** in small doses - For urinary retention, **bethanechol** (not carbachol) is the agent of choice when given systemically - Carbachol's systemic use is limited due to lack of selectivity and poor CNS penetration **High-Yield:** Carbachol ≠ systemic urinary retention agent. Bethanechol is the systemic cholinergic agonist for lower urinary tract dysfunction. ## Comparison Table: Cholinergic Agonists | Agent | Selectivity | BBB Penetration | Clinical Use | Route | | --- | --- | --- | --- | --- | | Pilocarpine | M3 selective | No | Glaucoma, xerophthalmia | Topical, oral | | Bethanechol | M3 selective | No | Urinary retention, GI motility | SC, oral | | Carbachol | Non-selective (M + N) | No | Glaucoma | Topical, IM/SC only | | Acetylcholine | Non-selective (M + N) | No | None (rapid hydrolysis) | — | [cite:KD Tripathi 8e Ch 6]