Final answer:
The largest comitant tropia (CT) with esotropia and a paretic eye fixating would likely be with the right eye fixating in an upward gaze, implicating the medial rectus as the underacting muscle, making choice (c) the correct answer.
Step-by-step explanation:
The question is about which eye and field of gaze produce the largest comitant tropia (CT), which happens when one eye has a muscle weakness (paretic eye) causing it to deviate when not fixating. The question also inquires about the underacting (UA) muscle in an esotropic condition, where the eyes turn inward. With esotropia, if it's a case of secondary deviation, when the paretic eye is fixating, CT is typically worse. Purely vertical eye movements are unusual and movements often include horizontal components, thus involving the medial and lateral rectus muscles. If the patient's problem is related to difficulty in abduction of the paretic eye, indicating lateral rectus muscle weakness, the underacting muscle for an esotrope would likely be the medial rectus muscle.
Considering the knowledge provided and based on the movements and muscle involvements, choice (c) Right eye; Upward gaze; Medial rectus seems to be the correct option because with upward gaze the inferior oblique is also involved, which is innervated by the oculomotor nerve like the medial rectus. The medial rectus muscle, when not functioning properly, can lead to esotropia. CT measurements tend to be larger in the upward gaze due to the increased demand on the inferior oblique muscle.