During a visit, Mitch, 2 years old, presents with large muscular calves and difficulty rising from a sitting position. Which lab test should you order?

During a visit, Mitch, 2 years old, presents with large muscular calves and difficulty rising from a sitting position. Which lab test should you order?

• A. Serum calcium
• B. Serum magnesium
• C. Serum phosphorus
• D. Serum creatine kinase

Answer: (D)

Proximal muscle weakness with calf pseudohypertrophy in a toddler points toward Duchenne muscular dystrophy, a condition where muscle fibers are damaged and release their contents into the blood. The best initial lab to order is serum creatine kinase because CK leaks out of damaged muscle fibers, and in Duchenne it becomes markedly elevated—even before substantial weakness is evident. This makes CK a sensitive screening test that can prompt timely genetic testing for the dystrophin gene and referral to neurology. The other electrolytes — calcium, magnesium, and phosphorus — don’t specifically reflect muscle breakdown and aren’t diagnostic for this myopathy.

Proximal muscle weakness with calf pseudohypertrophy in a toddler points toward Duchenne muscular dystrophy, a condition where muscle fibers are damaged and release their contents into the blood. The best initial lab to order is serum creatine kinase because CK leaks out of damaged muscle fibers, and in Duchenne it becomes markedly elevated—even before substantial weakness is evident. This makes CK a sensitive screening test that can prompt timely genetic testing for the dystrophin gene and referral to neurology. The other electrolytes — calcium, magnesium, and phosphorus — don’t specifically reflect muscle breakdown and aren’t diagnostic for this myopathy.