Among the following, which is associated with smooth pursuit in horizontal gaze nystagmus (HGN)?

Smooth pursuit is a type of eye movement where the eyes smoothly follow a moving object. In the context of horizontal gaze nystagmus (HGN), the assessment of smooth pursuit is critical for determining impairment.

The lack of smooth pursuit indicates that the eyes do not follow a moving object smoothly, but rather exhibit jerky or interrupted movements. This lack of smooth pursuit is a key indicator of potential impairment, especially related to the influence of certain drugs or alcohol.

The presence of sustained nystagmus refers to the observable continuous back-and-forth movement of the eyes when they are positioned to the side, suggesting that there may be central nervous system impairment. This finding also aligns with difficulties in smooth pursuit, reinforcing the idea that the presence of nystagmus can accompany the lack of smooth tracking ability.

The combination of both findings—lack of smooth pursuit and presence of sustained nystagmus—helps establish a pattern indicative of impairment. Thus, the correct answer reflects the association of both elements when assessing horizontal gaze nystagmus.