What description best defines nystagmus?

• A. A rapid, involuntary oscillation of the eyes with a slow phase in one direction and a fast corrective phase in the opposite direction
• B. A persistent dilation of the pupil
• C. A vergence of the eyes toward a near target
• D. A unidirectional sustained gaze

Answer: (A)

Nystagmus is a rhythmic, involuntary eye movement in which the eye drifts slowly in one direction (slow phase) and then makes a rapid corrective movement back in the opposite direction (fast phase). This combination of a slow drift with a fast corrective saccade is the defining feature. The other descriptions describe different eye conditions: pupil dilation is a change in pupil size, not an oscillating eye movement; vergence toward a near target is inward movement to maintain single vision at near, not an oscillation; and a unidirectional sustained gaze is simply holding the eyes in one direction without the characteristic back-and-forth movement.