Which factor can lead to increased nystagmus during the Horizontal Gaze Nystagmus (HGN) test?

The correct choice is based on the role of blood alcohol concentration (BAC) levels in influencing nystagmus. During the Horizontal Gaze Nystagmus (HGN) test, the examiner looks for specific physiological responses of the eyes, including involuntary jerking known as nystagmus, which can increase significantly with rising BAC levels.

As BAC levels increase, the central nervous system becomes more impaired, leading to greater difficulty in maintaining smooth pursuit movements of the eyes. This impairment manifests as an increased angle of onset of nystagmus and makes jerking more pronounced. It is a reliable indicator of potential impairment related to alcohol consumption.

Factors such as excess stress on the subject, exhaustion, or high ambient noise levels may affect a person's performance on the test, but they do not directly influence the physiological response of nystagmus in the same way that elevated BAC does. Thus, while these factors could potentially complicate the testing environment or the individual’s ability to focus, they are not primary causes of increased nystagmus during the HGN test.