Clinical reasoning: a 12-year-old boy with ascending weakness.

A 12-year-old boy presented with 3 weeks of calf pain, tripping, and progressive inability to walk. The onset was preceded by a sore throat 4 weeks prior, but no recent immunizations and no sick contacts. He began having problems "catching his toes" for 2 weeks. He had no visual complaints and no bowel or bladder incontinence. He had no recent travel and there were no heavy metal or solvent exposures. He had no prior medical history and he was on no prescription medications. Developmentally, he was on track and had just successfully completed fifth grade. However, he was reported to be behaviorally oppositional, especially regarding his diet which was restricted to beef jerky, yogurt from a squeeze tube, and fruit drinks. Family history included diabetic peripheral neuropathy in his mother, idiopathic peripheral neuropathy in his maternal grandfather, and left lower extremity neuropathy from trauma in his father. There was no known family history of recurrent pressure palsies or cardiac problems.