Review Question - QID 109485

A 3-year-old boy presents to the pediatrician for a well-child visit. His mother reports that his motor development seems to have slowed in the last year. At 2 years of age, the child was walking well and had started walking up and down stairs with help. He now seems to struggle with stairs and cries until one of his parents picks him up to go upstairs at bedtime. He also seems less interested in playing on playgrounds. The patient is otherwise healthy and enjoys going to preschool. The patient’s mother reports that one of her uncles used a wheelchair and died at a young age from an unknown disease. His temperature is 98.6°F (37°C), blood pressure is 98/62 mmHg, pulse is 135/min, and respirations are 26/min. On physical exam, the patient has symmetric weakness in the distal muscle groups of the lower extremities. His patellar and achilles reflexes are diminished bilaterally. The patient has the physical exam finding seen in Figure A. Which of the following mutations would result in the most severe phenotype of this disease?