A 35-year-old female schoolteacher from Delhi presents with a 6-week history of progressive hair loss and scaling on the scalp. Examination shows circular patches of alopecia with broken hairs at the margin, erythema, and fine scale. She reports mild pruritus. Wood's lamp examination shows a faint blue-green fluorescence. Pluck hair shows spores arranged in a sheath-like pattern around the hair shaft (endothrix pattern). What is the most likely causative organism?