A mother brings her 3-month-old baby to her pediatrician because he has progressively more listless for 3 days. The baby has been constiped and has refused to nurse or take a bottle. Upon physical examination, the physician notes that the baby has a sluggish papillary reaction to light, a week gag reflex, and poor anal sphincter control. The physician tentatively diagnosis this 3-month-old with infant botulism. Which test would be the most reliable for confirming this diagnosis?