USMLE Step 1 Practice Test 2

A 5-week-old girl is brought to the physician because of poor feeding, constipation, and excessive sleepiness for 2 weeks. She was born at term at home after an uncomplicated pregnancy, and no newborn screening was performed. The mother has no history of thyroid disease, takes no medications, and lives in an iodine-sufficient region. The infant has a hoarse cry. Examination shows a large, protruding tongue, dry and mottled skin, an umbilical hernia, decreased muscle tone, and a widely open posterior fontanelle. The skin and sclerae are jaundiced; serum total bilirubin concentration is 11 mg/dL, predominantly unconjugated. Which of the following is the most likely underlying cause of this patient's condition?