A mother brings her son to your office for evaluation of delayed growth, diarrhea, and pale, foul-smelling stools. The child has speech difficulties, recurrent infections requiring antibiotics, and vision problems, including difficulty seeing at night and bumping into objects. His movements are clumsy, resembling those of a “drunken man,” and he complains of tingling and burning pain in his hands and legs. The mother notes that some relatives have similar symptoms. On physical examination: Thin, ill-built individual with tremors in both hands. Gait: Irregular, clumsy, and jerky. Vision: 20/40 in both eyes, nystagmus, and dry skin. Neurological findings: Diminished tendon reflexes, loss of vibratory and position sense in the legs, loss of pain and temperature sensation, and muscle weakness. Other findings: Kyphoscoliosis, hepatomegaly, and acanthocytosis of red blood cells. Laboratory results reveal: Very low serum cholesterol, low triglycerides, and low-density lipoproteins. Elevated liver enzymes, anemia, and thorny-looking red blood cells. Of the following, what is the most likely diagnosis in this boy?
