Which condition involves the narrowing of the pupil?

The condition that involves the narrowing of the pupil is miosis. Miosis occurs when the iris sphincter muscle contracts, resulting in a smaller pupil size. This response can be triggered by various factors, such as increased light exposure, certain medications, or neurological conditions.

Understanding the other options provides further context: mydriasis refers to the dilation of the pupil, which is the opposite of miosis. Anisocoria describes a condition where there is unequal pupil size, which may arise from different underlying causes but does not involve pupil narrowing specifically. Nystagmus is characterized by involuntary eye movements and is unrelated to pupil size changes. Thus, miosis specifically denotes the narrowing of the pupil, distinguishing it clearly from these other conditions.