USMLE Question #493: Real Exam Question with Answer & Explanation

A 2-year-old boy is brought to the physician for a well-child examination. He was delivered at term after an uncomplicated pregnancy. His birth weight was 3500 g(7lb11oz), and Apgar scores were 8 and 10 at 1 and 5 minutes, respectively. At the age of 15 months, physical examination showed no abnormalities, but he was not yet talking. Both of his parents had learning difficulties in school, and his mother stopped attending after the 10 grade. He has a maternal uncle with cognitive th disabilities. He is at the 25 percentile for height, 15 percentile for weight, and 90 percentile for head circumference. He appears irritable, he resists making eye th th contact, and he is flapping his hands. Which of the following is the most likely cause of this patient's condition?