Review Question - QID 213661

A 2-year-old boy is brought in by his mother to the pediatrician for concerns about his development. The patient was last seen 1 year ago, after which the family had been lost to follow up. The mother reports that the patient’s head appears to be enlarged and that his body frame is smaller than that of his siblings at this age. Of note, his shoulders seem to be particularly “loose” and appear very narrow. On exam, the patient’s temperature is 98.3°F (36.8°C), blood pressure is 108/68 mmHg, pulse is 82/min, and respirations are 14/min. The patient is notable for having a widened skull with wide-set eyes. Both his anterior and posterior fontanelles are patent. The patient has a small upper jaw and does not yet have a full set of teeth. On further examination, the patient is notable for having underdeveloped clavicles, and the shoulders are hypermobile along the thoracic wall. Which of the following is the most commonly mutated gene with this condition?