A 60-year-old woman was hospitalized 5 days ago for treatment of a diabetic foot infection. On the first day in the hospital, clindamycin was initiated. The patient now complains of eight to nine watery stools per day, nausea, vomiting, and abdominal cramps relieved by defecation. On physical examination, skin turgor is decreased. Temperature is 38.6°C. Other vital signs are normal. > What diagnostic test is indicated in addition to fecal leukocytes and faecal occult blood?