A previously healthy 18-month-old girl presents to the emergency department with refusal to walk. Initially demonstrating only mild exercise intolerance, her weakness has progressed over the past 6 weeks, and she is now no longer able to stand. She has no ataxia, seizures, or oculobulbar weakness, and her language skills remain unaffected. Her growth and neurodevelopment were previously normal.

Further history reveals nonbilious, nonbloody vomiting since age 12 months, initially occurring once or twice per month but gradually increasing to several times per week. She has had no diarrhea, abdominal pain, or fever but has lost 1.0 kg (2.2 lb) of weight during the past month. Results of her remaining history and dietary review are unremarkable. Iron supplements were started empirically by her family physician and are her only medication.

On examination, she appears tired and will not stand without support. Her heart rate is 125 beats/min; respiratory rate, 26...

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