Soru

A $5$-month-old male infant is brought to the pediatric clinic by his parents for a well-child visit. The parents report that he is a "very quiet baby" who sleeps most of the day, but they are concerned because he frequently strains to pass hard, pellet-like stools. He is not yet rolling over and rarely coos. He was born at $39$ weeks gestation via uncomplicated vaginal delivery. The state's newborn screening program primarily measures thyroid-stimulating hormone (TSH) levels, which were reported as normal. On physical examination, his length is at the $5$ th percentile and weight is at the $25$ th percentile. His heart rate is $90$/min and temperature is $36.4^\circ\text{C}$ ($97.5^\circ\text{F}$). He exhibits generalized hypotonia and a stretched penile length of $1.5 \text{ cm}$ (normal $> 2.5 \text{ cm}$). Laboratory evaluation reveals:

Serum thyroid-stimulating hormone (TSH): $2.1 \mu\text{U/mL}$ (normal: $0.5$–$5.0 \mu\text{U/mL}$)
Serum free thyroxine (T4): $0.4 \text{ ng/dL}$ (normal: $0.9$–$2.2 \text{ ng/dL}$)
Serum glucose: $52 \text{ mg/dL}$

Which of the following is the most likely cause of this infant's condition?