**Core Concept**
Cholinomimetics are a class of drugs that mimic the action of the neurotransmitter acetylcholine in the parasympathetic nervous system. They stimulate muscarinic and nicotinic receptors, leading to various physiological responses, including increased salivation, bronchial secretions, and bladder contractions.

**Why the Correct Answer is Right**
Cholinomimetics are used to treat conditions that benefit from increased parasympathetic tone, such as glaucoma (by increasing aqueous humor outflow), myasthenia gravis (by enhancing neuromuscular transmission), and postoperative atony of the bladder (by increasing bladder contractions). However, in partial heart block, the use of cholinomimetics is contraindicated because they can further decrease heart rate and worsen heart block by increasing vagal tone.

**Why Each Wrong Option is Incorrect**
**Option A:** Glaucoma is a condition where cholinomimetics can be used to decrease intraocular pressure by increasing aqueous humor outflow.
**Option B:** Myasthenia gravis is a neuromuscular disorder where cholinomimetics can help enhance neuromuscular transmission and improve muscle strength.
**Option C:** Postoperative atony of the bladder is a condition where cholinomimetics can help increase bladder contractions and improve urinary retention.

**Clinical Pearl / High-Yield Fact**
When using cholinomimetics, it's essential to consider the potential for exacerbating conditions like partial heart block, and to monitor patients closely for signs of excessive vagal tone, such as bradycardia and hypotension.

**Correct Answer:**
✓ Correct Answer: D. Partial heart block