A 5-year-old girl from Delhi, who received only one dose of DPT vaccine at age 2 years (incomplete immunization), presents with a 4-day history of sore throat, fever, and a thick pseudomembrane over the tonsils. A Gram stain of the throat swab shows Gram-positive bacilli arranged in a characteristic pattern. Culture on Loeffler's medium shows small, greyish-white colonies with a "bread crumb" or "barber pole" appearance on methylene blue staining. The culture is negative on blood agar. Which is the most appropriate next step in management?