A 13-year-old boy comes to the office for evaluation of worsening scoliosis. He lost the ability to walk a year ago and has since been wheelchair bound. He has significant back pain and difficulty sitting in his wheelchair for extended periods. The patient also feels "too weak" to cough or take a deep breath. Review of medical records shows genetic studies confirming a mutation of the dystrophin gene on X chromosome p21. Histopathology of the patient's calf most likely indicates which of the following findings?

Question

A 13-year-old boy comes to the office for evaluation of worsening scoliosis.  He lost the ability to walk a year ago and has since been wheelchair bound.  He has significant back pain and difficulty sitting in his wheelchair for extended periods.  The patient also feels "too weak" to cough or take a deep breath.  Review of medical records shows genetic studies confirming a mutation of the dystrophin gene on X chromosome p21.  Histopathology of the patient's calf most likely indicates which of the following findings?

Quizplus · Accepted Answer

11ec160b_3a56_ef88_9249_f300b819cdf1_MD0001_00 Deletion of the dystrophin gene causes Duchenne muscle dystrophy (DMD). The condition is X-linked recessive and therefore affects primarily boys. Dystrophin is a structural protein of muscle fibers, and its absence causes muscle fiber destruction (myonecrosis). Variation in muscle fiber shape and size, regenerating fibers, and increased amounts of connective tissue are seen on light microscopy. Disease onset is age 2-5. Muscles of the proximal lower extremities, back, and pelvic and shoulder girdles are affected first. Symptoms of DMD include the following: Most patients with DMD are wheelchair bound by age 12. Scoliosis progresses rapidly due to muscle imbalance and body positional changes. Worsening scoliosis is complicated by restrictive pulmonary function (decreased vital capacity and total lung capacity). (Choice B) Hypertrophy of muscle fibers may occur in the early stages of DMD. However, fibrofatty change of the enlarged muscles would be expected in a patient at this advanced stage of the disease. (Choice C) Lipid accumulation within muscle fibers is seen in lipid myopathies such as carnitine palmitoyltransferase deficiency. (Choice D) Prominent inflammatory infiltrate is a characteristic histologic feature of polymyositis and dermatomyositis. These conditions cause proximal muscle weakness without distal pseudohypertrophy. (Choice E) Ragged red fibers refer to muscle fibers with irregular contours and a blotchy red appearance. They are seen in mitochondrial myopathies. Educational objective: Duchenne muscle dystrophy manifests with proximal muscle weakness and atrophy. True hypertrophy of the distal muscle is noted early in the disease as distal muscles compensate for weak proximal ones. Muscle fibers of the distal extremities are later replaced by fat and connective tissue (pseudohypertrophy). __________ References: Muscle disease. (http://www.ncbi.nlm.nih.gov/pubmed/24488829) Entries in the Leiden Duchenne muscular dystrophy mutation database: an overview of mutation types and paradoxical cases that confirm the reading-frame rule. (http://www.ncbi.nlm.nih.gov/pubmed/16770791)

A 13-Year-Old Boy Comes to the Office for Evaluation of Worsening