The mother of a 2-week-old infant reports that since birth, her infant sleeps most of the day; she has to awaken her every 4 hours to feed, and she will take only an ounce of formula at a time. She also is concerned that the infant has persistently hard, pellet-like stools. On your examination you find an infant with normal weight and length, but with an enlarged head. The heart rate is 75 beats per minute and the temperature is 35degC (95degF). The child is still jaundiced. You note large anterior and posterior fontanelles, a distended abdomen, and an umbilical hernia. This clinical presentation is likely a result of which of the following?