Rapid Review: Vitamin Deficiencies

Sample question:

A 2-year-old boy is brought by his parents to the clinic because of opacity and hyperpigmentation of both eyes. His skin is noted to be dry and scaly, especially on the arms, legs, shoulders, and buttocks. He also gets sick frequently. Physical examination reveals an afebrile and weak-looking child. There is a note of opacity and keratinization of the cornea with dry, scaly layers, and plaque. Which of the following is the most likely diagnosis?