PLE-3492

An 8-year-old boy presented to you with acute onset of fever, cough, sore throat, painful swallowing, and hoarseness. Physical examination showed tender cervical lymph nodes and adherent membranes over his tonsils. You then performed a throat swab to confirm your diagnosis. Microscopic examination of the isolated organism after culturing your specimen would yield characteristic metachromatic granules when stained with which of the following substances?