A 73-year-old woman in a nursing home develops profuse watery diarrhoea after a course of clindamycin. Stool test is positive for Clostridioides difficile toxin. She is haemodynamically stable, with creatinine 90 µmol/L and white blood cell count 11 × 10⁹/L. According to AMMI Canada guidelines, what is the preferred initial therapy for this non-severe episode?