A 2-month-old infant boy is evaluated for the first time since delivery. The patient was born at 40 weeks gestation via vaginal delivery
and had a normal prenatal and postnatal
birth history. He feeds 2-3 oz of breastmilk and formula every 2-3 hours. He is in the 75th
percentile for height, 75th
percentile for weight, and 50th
percentile for head circumference. On physical examination, there is prominent head
posturing. The head is tilted to the right side, and the chin is rotated to the left shoulder. A mass is palpable in the inferior portion of the neck on the right side. The mass is firm and does not transilluminate with a penlight. The rest of the physical examination, including the reflexes
and muscular tone of the extremities, is normal. Which of the following is the most likely cause of this patient’s condition?