<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 10-year-old boy is brought to the physician because of recurring episodes of achy muscle pain in his legs. He has a history of poor school performance despite tutoring and has been held back two grades. He is at the 40th percentile for height and 30th percentile for weight. Examination shows ptosis, a high-arched palate, and muscle weakness in the face and hands; muscle strength of the quadriceps and hamstrings is normal. Sensation is intact. Percussion of the thenar eminence causes the thumb to abduct and then relax slowly. Which of the following is the most likely underlying cause?? A: Apoptosis of lower motor neurons, B: Defect of voltage-gated sodium channels of the sarcolemmal membrane, C: Complete impairment of the dystrophin protein, D: Humoral immune attack against the endomysial blood vessels, E: CTG trinucleotide expansion in the DMPK gene,  <response>: This is a clinical vignette describing a 10-year-old boy with recurring episodes of muscle pain and weakness in his legs, face, and hands. He also has poor school performance, short stature, high-arched palate, and ptosis. Percussion of the thenar eminence shows slow relaxation of the thumb abductor muscle. The key findings are proximal and distal muscle weakness, ptosis, high-arched palate, and myotonic response to percussion of the thenar eminence. This clinical presentation is most consistent with myotonic dystrophy type 1, which is caused by CTG trinucleotide repeat expansion in the DMPK gene.Choices A and B can cause muscle weakness but do not explain the other findings. Choice C causes proximal muscle weakness but not the distal and facial weakness seen here. Choice D causes muscle inflammation but not the myotonic response. Only choice E explains all the clinical findings in this vignette.E: CTG trinucleotide expansion in the DMPK gene 