During fundoscopic examination, a patient is given phenylephrine eye drops to achieve mydriasis. Unlike tropicamide, phenylephrine does not cause cycloplegia. The mechanism underlying this difference is that phenylephrine acts by:

• A Blocking M3 receptors on the ciliary muscle
• B Stimulating alpha-1 receptors on the radial (dilator) muscle of the iris ✓
• C Inhibiting acetylcholinesterase at the pupillary sphincter
• D Stimulating beta-2 receptors on the ciliary epithelium

Explanation

Phenylephrine is a selective alpha-1 agonist that directly stimulates alpha-1 adrenoceptors on the radial (dilator) muscle of the iris, causing pupillary dilation without affecting the ciliary muscle. Cycloplegia requires M3 receptor blockade (as with tropicamide) on the ciliary muscle, which phenylephrine does not produce, preserving accommodation.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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Written and medically reviewed by the StethoPrep medical team.