A 4-Year-Old Boy Is Seen by the Pediatrician for Clumsiness

A 4-year-old boy is seen by the pediatrician for clumsiness and difficulty climbing stairs.Physical examination shows poorly developed muscles, and a blood test shows that the level of the muscle enzyme creatine kinase is 40 times the upper limit of normal.This disease is most likely caused by a deficiency of the muscle protein:

A) Troponin.
B) Tropomyosin.
C) -Actinin.
D) Dystrophin.
E) Titin.