**Question:** Injection of muscarinic agonist in conjunctival sac will lead to all of the following except-

A. Inhibition of lacrimation
B. Increased intraocular pressure
C. Reduced corneal reflex
D. Enhanced visual acuity

**Core Concept:**
Muscarinic agonists are drugs that activate muscarinic receptors, which are G-protein coupled receptors found in various parts of the body. In the context of the eye, these receptors are primarily found in the lacrimal gland, cornea, and ciliary body. Muscarinic agonists are used to stimulate these receptors and induce their physiological responses.

**Why the Correct Answer is Right:**
The correct answer is "D. Enhanced visual acuity" because muscarinic agonists primarily affect the lacrimal gland, cornea, and ciliary body, not the visual system. Stimulation of these receptors leads to the following responses:

1. **Inhibition of lacrimation (Option A):** Muscarinic agonists inhibit lacrimation by stimulating the lacrimal gland to produce less tear production, which helps to reduce watering of the eyes.
2. **Increased intraocular pressure (Option B):** Muscarinic agonists can increase intraocular pressure by stimulating the ciliary body, which is responsible for the production of aqueous humor. A rise in aqueous humor production can lead to an increase in intraocular pressure.
3. **Reduced corneal reflex (Option C):** Muscarinic agonists can reduce the corneal reflex, as stimulation of the corneal receptors can lead to a decrease in corneal sensitivity and the corneal reflex.

**Why Each Wrong Option is Incorrect:**
4. **Enhanced visual acuity (Option D):** Muscarinic agonists have no direct impact on visual acuity. Although they might indirectly affect visual acuity through lacrimation and intraocular pressure changes, the primary effects listed above (inhibition of lacrimation and increased intraocular pressure) are more relevant to ocular function and do not directly correlate with visual acuity improvement.

**Core Concept:**
Muscarinic agonists primarily affect lacrimal glands, ciliary body, and cornea, leading to changes in lacrimation, intraocular pressure, and corneal reflex, but have no direct impact on visual acuity.

**Clinical Pearls:**
1. In clinical practice, muscarinic agonists are used to treat dry eyes and glaucoma. However, their effects on visual acuity are not a primary goal of therapy.
2. The main effects of muscarinic agonists are to stimulate lacrimal gland secretion, reduce aqueous humor production, and modulate the corneal reflex, which are essential for maintaining ocular health but not directly related to visual acuity.