In a 30-year-old woman with lower abdominal pain, irregular menstrual cycles, and a tender, enlarged uterus on bimanual examination, what is the most likely diagnosis?

Most Likely Diagnosis: Uterine Fibroid (C)

In a 30-year-old woman presenting with lower abdominal pain, irregular menstrual cycles, and a tender, enlarged uterus on bimanual examination, the most likely diagnosis is uterine fibroid (leiomyoma).

Clinical Reasoning

Why Uterine Fibroid is Most Likely

Uterine fibroids are the most common benign tumor in patients of reproductive age 1. The clinical presentation matches the classic triad of fibroid symptoms:

• Irregular menstrual cycles - Fibroids commonly cause heavy and prolonged uterine bleeding 1
• Lower abdominal pain - Pelvic pain is a cardinal symptom of fibroids 1
• Enlarged uterus - Physical examination revealing an enlarged uterus is characteristic, as fibroids arise from myometrial smooth muscle 1

The tenderness on examination can occur with fibroids, particularly if there is rapid growth, degeneration, or associated bulk symptoms 1.

Differentiating from Other Options

Endometriosis (Option B): While endometriosis causes pelvic pain and dysmenorrhea 2, 3, it typically does not cause significant uterine enlargement on bimanual examination. Endometriosis involves ectopic endometrial tissue outside the uterus 4, so the uterus itself remains normal-sized unless there is concurrent adenomyosis.

Adenomyosis (not listed but relevant): Adenomyosis does cause an enlarged, tender uterus with heavy menstrual bleeding and pelvic pain 2, 5. However, it is more common in older reproductive-age women (typically 35-50 years) and often occurs in multiparous women. At age 30, fibroids remain statistically more likely 1.

Ovarian Cyst (Option A): Ovarian cysts would present as an adnexal mass separate from the uterus on bimanual examination, not as an enlarged uterus itself. The examination findings clearly indicate uterine pathology.

Ectopic Pregnancy (Option D): While ectopic pregnancy must always be ruled out in reproductive-age women with abdominal pain, it does not cause a uniformly enlarged uterus. The uterus may be slightly enlarged due to hormonal changes, but the key finding would be an adnexal mass with tenderness, not a diffusely enlarged uterus. Additionally, irregular cycles rather than amenorrhea make this less likely.

Diagnostic Approach

Immediate next steps:

1. Pregnancy test - Always rule out pregnancy first in any reproductive-age woman with abnormal bleeding 1
2. Hemoglobin level - Assess for anemia from chronic blood loss 1
3. Pelvic ultrasound - Transvaginal ultrasound is the initial imaging modality to confirm fibroids and characterize their location, size, and number 1, 6

If ultrasound is inconclusive:

• MRI pelvis - Provides superior tissue characterization and multiplanar visualization, particularly useful when ultrasound cannot completely evaluate the uterus due to patient body habitus or multiple fibroids 6

Critical Pitfalls to Avoid

• Do not assume benign pathology without imaging confirmation - While fibroids are most likely, imaging is essential to rule out other structural causes 6
• Always obtain a pregnancy test first - Ectopic pregnancy can be life-threatening and must be excluded 1
• Consider endometrial sampling if risk factors present - Though less likely at age 30, if the patient has risk factors for endometrial cancer (obesity, chronic anovulation, prolonged unopposed estrogen), endometrial biopsy may be indicated 6, 7