You evaluate a patient with unusual eye findings including restricted upward gaze. When he tries to look up, his eyes seem repeatedly to converge and retract deeper into his sockets...

You evaluate a patient with unusual eye findings including restricted upward gaze. When he tries to look up, his eyes seem repeatedly to converge and retract deeper into his sockets (convergence-retraction nystagmus). His pupils do not constrict in response to light but do constrict when he focuses on near objects, like the tip of his nose. Which of the following structures is most likely being compressed?

A. Ventral midbrain

B. Dorsal midbrain

C. Oculomotor nerve

D. Optic nerve

E. Pituitary gland