Which cranial nerve is assessed by brushing a wisp of cotton against the eye?

The cranial nerve assessed by brushing a wisp of cotton against the eye is indeed the fifth cranial nerve, known as the trigeminal nerve. This nerve has a sensory function that includes sensation from the face, including the cornea of the eye. Specifically, when the cornea is stimulated by the cotton wisp, it is usually the sensory fibers that convey sensations back to the brain, prompting a reflexive blink response, which is part of the corneal reflex.

This assessment is critical in evaluating sensory function. In contrast, the other nerve options listed have different primary roles: the second cranial nerve (optic nerve) primarily handles vision; the third cranial nerve (oculomotor nerve) is involved in controlling most of the eye's movements and pupil constriction; and the fourth cranial nerve (trochlear nerve) is responsible for the movement of the superior oblique muscle of the eye. These functions are distinct from the sensory function provided by the trigeminal nerve, highlighting the significance of CN V in this scenario.