What is the most likely etiology of a dilated pupil that does not constrict with both 0.125% and 1% Pilocarpine?

• A. Adie's Tonic Pupil
• B. CN III Palsy
• C. Horner's Syndrome
• D. Pharmacological Dilation

Answer: (D)

The key idea is that pilocarpine tests rely on muscarinic activation of the iris sphincter. When a pupil remains dilated and fails to constrict to both very dilute (0.125%) and standard (1%) pilocarpine, the sphincter muscle cannot respond to cholinergic stimulation at all. That pattern points to a pharmacologic block of the muscarinic receptors—most commonly from anticholinergic exposure blocking the receptor site. In contrast, a tonically dilated pupil from Adie’s shows supersensitivity to very dilute pilocarpine and would constrict to 0.125%, not remain unmoved. CN III palsy could leave the muscle responsive to direct receptor agonists or show variable pilocarpine responses, but the complete lack of constriction at both concentrations most strongly indicates pharmacologic dilation.