Q. A 7-year-old boy was brought in by his parents due to the recent emergence of various symptoms. Over the past few months, his parents noticed that he had difficulty seeing after sunset, often stumbling over objects in low light. They also noticed that his skin became rougher and he had small bumps on the back of his arms. Additionally, the boy had been missing school frequently due to recurrent upper respiratory tract infections. He was previously in the 50th percentile for his weight and height but has now slipped below the 10th percentile. The family had migrated from a region known for malnutrition, and their diet primarily consisted of rice and lentils with limited access to fresh fruits and vegetables. Upon further examination, you notice pale conjunctiva and some grey, foamy patches on the bulbar conjunctiva. Given the patient's symptoms and history, which of the following vitamin deficiencies is the most likely cause of his condition?