A 35-year-old nulliparous woman undergoes routine pelvic ultrasound and is found to have a 3 cm unilateral ovarian mass with a smooth contour and no internal vascularity on Doppler. The mass appears cystic with a thin wall. Serum tumor markers (AFP, β-hCG, inhibin) are normal. Which investigation is most appropriate to further characterize this lesion and guide management?