What is the criterion distinguishing a dominant follicle from a simple cyst in ovarian ultrasound?

• A. Dominant follicle: <25 mm; Simple cyst: >25 mm
• B. Dominant follicle: >25 mm; Simple cyst: <25 mm
• C. Dominant follicle: 25-30 mm; Simple cyst: 30-40 mm
• D. Dominant follicle: <10 mm; Simple cyst: >40 mm

Answer: (A)

In ovarian ultrasound, the key difference between a dominant follicle and a simple cyst is size, because both can look like fluid-filled sacs. The dominant follicle is the growing follicle toward ovulation and is typically under 25 millimeters in diameter. If the structure enlarges beyond about 25 millimeters, it’s considered a simple ovarian cyst rather than the dominant follicle. This size threshold helps distinguish a normal physiologic follicle from a cystic lesion that may require follow-up. Visually, both can be fluid-filled and thin-walled with posterior acoustic enhancement, but the clinical interpretation rests on diameter and the cycle context. Doppler can help in broader assessment: a corpus luteum shows vascularity, whereas a simple cyst usually lacks internal flow. The takeaway is that 25 mm serves as the practical cutoff separating a dominant follicle from a simple cyst in this ultrasound context.