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?

Nemaline rod myopathy, one form of congenital myopathy, may present in infancy or childhood and maybe nonprogressive or slowly progressive. Congenital myopathies are often named for their characteristic histologic features. Channelopathies typically present as periodic paralysis with abnormalities in serum potassium. The two glycogen storage diseases most often affecting striated muscle include Pompe disease (type II) and McArdle disease (type V). There are increased glycogen deposits seen with periodic acid-Schiff (PAS) stain. Neuropathies affect muscle through denervation, leading to groups of atrophic muscle fibers. Polymyositis and dermatomyositis are inflammatory myopathies that are accompanied by myalgia and fiber degeneration with increased serum creatine kinase. Mitochondrial myopathies may appear in childhood but are usually progressive, and often other organs such as the heart or brain are involved.