Torticollis

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 75^th percentile for height, 75^th percentile for weight, and 50^th 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?