A 28-year-old woman presents with a 2-month history of circular, well-demarcated, scaly patches on her scalp with partial alopecia. She reports mild pruritus and denies systemic symptoms. Examination reveals multiple lesions with central clearing and an active inflammatory border. KOH mount from plucked hairs and scale shows septate hyphae. What is the most appropriate next investigation to identify the causative organism and guide treatment?