Which drug is a sympathomimetic used to dilate the pupil?

• A. Tropicamide
• B. Phenylephrine hydrochloride
• C. Cyclopentolate
• D. Pilocarpine

Answer: (B)

Dilation of the pupil is achieved by activating the sympathetic pathway that controls the iris dilator muscle. A direct sympathomimetic used for this effect is phenylephrine, an alpha-1 adrenergic agonist. By stimulating these receptors on the radial muscle of the iris, it causes the muscle to contract and the pupil to enlarge, without affecting accommodation.

The other drugs work through different mechanisms: Tropicamide and cyclopentolate are anticholinergic agents that block muscarinic receptors, leading to dilation and often some loss of focusing ability (cycloplegia). Pilocarpine, in contrast, is a muscarinic agonist that stimulates the sphincter muscle to constrict the pupil (miosis). That's why phenylephrine is the correct choice when specifically asking for a sympathomimetic used to dilate the pupil.