Idiopathic hirsutism is the most likely diagnosis because this patient has isolated hirsutism with normal vital signs, normal BMI, and regular menstrual cycles , suggesting preserved ovulation and no clinically significant endocrine disturbance. MCCQE objectives emphasize that the most common causes of hirsutism are PCOS and idiopathic hirsutism; PCOS typically includes menstrual irregularity/oligo-ovulation and often metabolic features (overweight, insulin resistance), none of which are present here. Androgen-secreting ovarian tumors (hilar cell or Sertoli–Leydig cell tumors) usually cause rapid onset, severe hyperandrogenism and virilization (deepening voice, clitoromegaly, marked acne) and often disrupt menses—features not described. Nonclassic congenital adrenal hyperplasia can present with hirsutism, but it more commonly associates with acne, infertility, or menstrual abnormalities and requires biochemical suspicion rather than being the “most likely” with normal cycles. Therefore, idiopathic hirsutism—often due to increased peripheral androgen sensitivity or local 5-alpha reductase activity—is the best fit.