An ophthalmology trainee instils phenylephrine 2.5% drops into a patient's right eye before fundus examination. The pupil dilates widely, but the light reflex remains intact and the patient can still focus on near objects. This pattern occurs because phenylephrine acts on:

• A Muscarinic receptors of the sphincter pupillae
• B Alpha-1 receptors of the dilator pupillae ✓
• C Beta-2 receptors of the ciliary epithelium
• D Nicotinic receptors of the ciliary ganglion

Explanation

Phenylephrine is a direct alpha-1 agonist acting on the radial dilator pupillae, producing mydriasis without cycloplegia. Because the sphincter pupillae and ciliary muscle (both muscarinically innervated) are untouched, the light reflex and accommodation remain intact. Atropine would block M3 receptors and cause mydriasis WITH loss of accommodation, which distinguishes it from phenylephrine in this scenario.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

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Written and medically reviewed by the StethoPrep medical team.