Question:

A 3-year-old boy is brought to the office due to difficulty keeping up with the other children at daycare. Over the last 2 months, the daycare attendants have noticed that the patient has seemed more tired and less interested in active play. He has also had difficulty climbing stairs and jumping. The patient's parents mention that he is still eating well and gaining weight but has had recurrent upper respiratory infections since starting daycare. His last upper respiratory infection began 5 days ago.

• Temperature is 37°C (98.6°F), blood pressure is 90/50 mm Hg, pulse is 94/min, and respirations are 18/min.

• Examination shows clear rhinorrhea bilaterally. Cardiopulmonary and abdominal examinations are normal.

• Neurologic examination demonstrates 1+ patellar and Achilles reflexes bilaterally. The calves are enlarged and nontender.

Which of the following is the most likely cause of this patient's presentation?

Options:

• A. Absent dystrophin protein expression

• B. Acetylcholine receptor antibody production

• C. Muscle fiber viral inflammation

• D. Neurotransmitter neurotoxin blockade

• E. Peripheral nerve axon immune cross-reactivity

Correct Answer: A. Absent dystrophin protein expression

🧠 Genetic Mutation in DMD Gene on X-Chromosome
⬇️
Lack of Dystrophin Protein Production
Dystrophin normally stabilizes muscle cell membranes during contraction
⬇️
🧠 Loss of Membrane Stability in Muscle Cells
⬇️
Calcium Influx into Muscle Cells
⬇️
Calcium-Dependent Enzymes Activation (e.g., Calpains)
⬇️
Progressive Muscle Fiber Damage and Necrosis
⬇️
🧠 Inflammation and Repair with Fatty and Fibrotic Tissue
⬇️
Pseudohypertrophy of Calf Muscles

Enlarged calves observed due to fat and fibrotic tissue replacing damaged muscle

⬇️
Progressive Weakness in Proximal Muscles (e.g., Hips, Thighs)
⬇️
Difficulty Climbing Stairs, Running, and Rising from the Floor

Proximal muscle weakness affects activities that require large muscle groups

⬇️
Reduced Reflexes (1+ in Patellar and Achilles)

Loss of muscle integrity reduces reflex response

⬇️
Progressive Muscle Degeneration
⬇️
🧠 Increased Vulnerability to Respiratory Muscle Weakness

Diaphragm and intercostal muscles may weaken over time

⬇️
Recurrent Respiratory Infections

Weakened respiratory muscles impair effective cough and clearance of secretions

Explanation of Incorrect Options:

B. Acetylcholine Receptor Antibody Production

• Associated with Myasthenia Gravis; would cause fluctuating weakness, not persistent proximal muscle weakness and calf pseudohypertrophy

C. Muscle Fiber Viral Inflammation

• Associated with viral myositis; presents as acute muscle pain and weakness, not the gradual progressive weakness seen in DMD

D. Neurotransmitter Neurotoxin Blockade

• Seen in botulism; causes descending paralysis, autonomic dysfunction, and cranial nerve involvement, not pseudohypertrophy or proximal weakness

E. Peripheral Nerve Axon Immune Cross-Reactivity

• Seen in Guillain-Barré Syndrome; would cause ascending paralysis with areflexia, not selective proximal muscle weakness and pseudohypertrophy

Flashcards

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