If a patient is unable to distinguish the largest letter on a Snellen chart, the assistant should:

The choice to determine if the patient can count fingers at a given distance is the most appropriate action following the inability to read the largest letter on a Snellen chart. This step provides a further evaluation of the patient's visual capability, moving from the recognition of letters to a more basic and quantifiable test. Counting fingers can help to ascertain a rough estimation of visual acuity, as patients may still have some visual perception even if they cannot recognize letters.

This method allows the healthcare provider to gauge what level of vision the patient has, thus enabling better planning for further assessments or interventions. If a patient cannot count fingers, it indicates a more significant impairment of vision, leading to the next steps in evaluation.

In contrast, assessing for hand motion is a more advanced step that would generally follow the inability to count fingers. Checking for light perception is also a further step in evaluating vision and may not be necessary if the patient can still count fingers. Recording "unable to assess" does not provide valuable information for the patient's ongoing care, as it does not help in understanding the extent of the visual impairment. Therefore, counting fingers strikes the right balance between assessing visual function while being practical in the timeframe of a patient examination.