What is the most likely diagnosis for a 5-cm left adnexal mass in a woman with a recent menstrual period and no symptoms?

Most Likely Diagnosis: Follicular Cyst

The most likely diagnosis is a follicular cyst (Option A), as this represents the most common functional ovarian mass in premenopausal women, particularly when the patient is asymptomatic with a recent menstrual period and negative pregnancy test. 1

Clinical Reasoning

Why Follicular Cyst is Most Likely

• Most simple cysts in premenopausal women are functional in nature (follicular or corpus luteum cysts) and will resolve spontaneously 1
• The 5-cm size falls within the typical range for functional cysts that can be observed 2, 3
• The absence of symptoms strongly suggests a benign functional process rather than complications like torsion or rupture 1
• The recent menstrual period indicates active ovarian cycling, making functional cysts highly probable 2

Excluding Other Options

Corpus Luteal Cyst (Option D):

• While also a functional cyst, corpus luteal cysts typically form after ovulation in the luteal phase of the menstrual cycle 2
• The timing "a few weeks" after her last menstrual period could represent either follicular or luteal phase, but follicular cysts are more common overall in premenopausal women 1
• Corpus luteal cysts often have characteristic sonographic features including spiderweb-appearing or retracting clot with peripheral vascularity, which would be noted on ultrasound if present 1

Ectopic Pregnancy (Option C):

• Definitively ruled out by the negative urine pregnancy test 2, 3
• Ectopic pregnancy must always be excluded in women of reproductive age with adnexal masses, but the negative test eliminates this diagnosis 3, 4

Pelvic Kidney (Option B):

• This is a congenital anomaly that would likely have been detected earlier in life 2
• Ultrasound would clearly differentiate a solid renal structure from an ovarian cystic mass 1
• This is an uncommon finding and not the "most likely" diagnosis in this clinical scenario 2

Diagnostic Approach

Initial Evaluation

• Transvaginal ultrasound is the essential imaging modality for characterizing adnexal masses in premenopausal women 1
• Color or power Doppler should be included to assess for internal vascularity and differentiate solid from cystic components 1
• Simple cysts (unilocular, anechoic, smooth walls, no solid components) have less than 0.4% risk of malignancy in premenopausal women 1

Management Strategy

• Asymptomatic premenopausal patients with simple ovarian cysts less than 10 cm can be observed with follow-up ultrasound in 8-12 weeks 2, 3
• Approximately 70% of functional cysts will resolve spontaneously within this timeframe 1
• No contraceptive use means the cyst is not suppressed, but observation remains appropriate for simple cysts 2

Important Clinical Pitfalls

• Do not assume all 5-cm masses require immediate surgery—most functional cysts in premenopausal women are benign and resolve spontaneously 1
• Always obtain pregnancy test before assuming a functional cyst diagnosis, as ectopic pregnancy can present similarly 2, 3
• Ensure adequate ultrasound characterization with both grayscale and Doppler evaluation to identify any concerning features (solid components, papillary projections, irregular walls) that would suggest non-functional etiology 1
• Size alone does not determine malignancy risk—morphologic features on ultrasound are far more important than size in premenopausal women 1