Step 1: Recall the classic triad.
Horner syndrome comes from an interruption of the cervical sympathetic chain, so the eye loses sympathetic drive
Step 2: Link the anatomy to the signs.
Loss of sympathetic tone drops the eyelid, ptosis; shrinks the pupil, miosis; and stops facial sweating on that side, anhidrosis
Step 3: Rule out the distractors.
Mydriasis and hyperhidrosis are the opposite of what a sympathetic block causes, and proptosis is not a feature of this syndrome
\[ \boxed{Ptosis + Miosis + Anhidrosis} \]