<s>[INST] context: The ophthalmic artery is a branch of the internal carotid artery and provides origin to the ocular and orbital vessels, including the central artery of the retina, which supplies the retina. The central artery of the retina is an end artery that has no anastomoses with other arterial sources; therefore, occlusion of this artery will result in loss of vision. The nasociliary nerve is a branch of the ophthalmic nerve. It is the general sensory nerve for the eye and is the afferent limb of the corneal blink reflex; it has no direct effect on vision. The anterior ethmoidal nerve is a branch of the nasociliary nerve and supplies the anterior ethmoid air cells, the nasal septum, and the lateral walls of the nasal cavity; it also supplies the skin on the bridge of the nose. The trochlear nerve is the fourth of the 12 cranial nerves and innervates the superior oblique muscle, one of the six extraocular muscles. The extraocular muscles function in the movement of the eyeball and not the perception of light. The optic nerve is the second of the 12 cranial nerves and is responsible for vision. A lesion of the optic nerve would lead to blindness; however, based on the location of the patient's infection, the optic nerve was not affected by the loss of arterial supply. [/INST] question: A 22-year-old man is admitted to the emergency department with a sinus infection. Radiographic examination reveals posterior ethmoidal cell infection. During physical examination the patient complains of progressive loss of vision. Which of the following structures is most likely affected? | options: A: Ophthalmic artery B: Nasociliary nerve C: Anterior ethmoidal nerve D: Trochlear nerve | answer: A </s>