Which of the following is a symptom of paralytic strabismus?

The symptom of paralytic strabismus is best characterized by the deviation of the eyes due to muscle or nerve issues. In paralytic strabismus, one or more muscles responsible for eye movement are affected, often due to a nerve palsy or another type of dysfunction, leading to an inability to coordinate eye movements properly. This condition results in misalignment where the affected eye cannot align with the other eye for proper focus, which is a hallmark of the disorder.

The other options, while they may indicate problems with eye function, do not specifically define the central issue associated with paralytic strabismus. Involuntary eyelid closure is more indicative of conditions such as blepharospasm, difficulty crossing the eyes does not directly signify a nerve or muscle issue specifically related to strabismus but could relate to general eye muscle function issues, and chronic headaches are often a consequence of visual disturbances or strain rather than a direct symptom of strabismus itself. Therefore, the deviation stemming from muscle or nerve issues is the symptom that aligns most accurately with paralytic strabismus.