Final answer:
The diagnosis for a patient with double vision upon looking to the left, where the right eye does not adduct and there is nystagmus in the left eye, is internuclear ophthalmoplegia. This condition results from a lesion affecting the medial longitudinal fasciculus, interrupting the coordination of eye movements.
Step-by-step explanation:
The patient presenting with double vision when looking to the left, where the right eye fails to adduct and there is nystagmus in the left eye, likely has a condition known as internuclear ophthalmoplegia (INO). This condition occurs when there's a lesion in the medial longitudinal fasciculus (MLF), which is responsible for coordinating eye movements, particularly the horizontal conjugate gaze. INO is characterized by the failure of one eye to adduct during horizontal gaze toward the opposite side while the other eye abducts with nystagmus. Diplopia, or double vision, is often the result when the two eyes are not perfectly aligned due to the failure of one of the extraocular muscles to function properly, and this can be a symptom of various underlying causes including INO.