## Core Concept
Cholinergic drugs, which mimic the action of the neurotransmitter acetylcholine, are used in various clinical conditions. They can stimulate both the muscarinic and nicotinic receptors, depending on their specificity. The therapeutic applications of cholinergic drugs are diverse, ranging from treating glaucoma to enhancing gastrointestinal motility.

## Why the Correct Answer is Right
Cholinergic drugs are beneficial in conditions like glaucoma (by reducing intraocular pressure), myasthenia gravis (by enhancing neuromuscular transmission), and in some cases of gastrointestinal disorders (by increasing motility). However, in conditions where there's already an excess of cholinergic activity or where increasing cholinergic activity could be detrimental, these drugs are not used.

## Why Each Wrong Option is Incorrect
- **Option A:** Glaucoma - Cholinergic drugs can be used to treat glaucoma by decreasing intraocular pressure, making this not the correct answer.
- **Option B:** Myasthenia gravis - Cholinergic drugs, specifically anticholinesterases, are used to treat myasthenia gravis by increasing neuromuscular transmission, so this is not the correct answer.
- **Option C:** Paralytic ileus - Cholinergic drugs can stimulate gastrointestinal motility and might be considered in conditions like paralytic ileus, although they're not the first line, making this not the best choice.

## Why Option D is Correct
- **Option D:** Asthma - Cholinergic drugs, by stimulating muscarinic receptors in the airway, can cause bronchoconstriction. Therefore, they are generally contraindicated in asthma, a condition characterized by episodic bronchoconstriction and inflammation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that cholinergic drugs can have significant effects on the cardiovascular system, including bradycardia and hypotension, and on the respiratory system, particularly bronchoconstriction. This makes their use in asthma and similar obstructive airway diseases risky without proper management.

## Correct Answer: D. Asthma