A 10-year-old girl has exhibited muscular weakness since early childhood that has not worsened. She can ambulate unassisted but does not participate in strenuous physical activities. On examination, she has 4/5 motor strength in proximal muscles and 5/5 in distal muscles. There is no muscle pain on palpation. A biopsy of the deltoid muscle is obtained, and with Gomori trichrome stain, microscopic analysis shows subsarcolemmal aggregates of rod-shaped intracytoplasmic inclusions. Laboratory studies show a normal serum creatine kinase. Which of the following is the most likely form of muscle disease she has?