A 14-year-old male high school student presents with 3 days of progressive weakness in all limbs, worsening over 2 hours with respiratory distress. Examination shows poor respiratory effort, weak voice, normal bowel/bladder function. RR 32/min, intact cranial nerves, limb muscle strength grade 1, absent tendon reflexes, normal sensation, negative pathological signs. Serum K+ 3.8mmol/L, CSF: WBC 5×10<sup>6</sup>/L, protein 0.45g/L, glucose 3.0mmol/L, chloride 122mmol/L. The most likely diagnosis is: