Differential Diagnosis for Left Lower Quadrant Abdominal Pain
- Single most likely diagnosis
- Ovarian cyst rupture or torsion: The presence of a 4-cm cyst with thin walls on the left ovary, along with sudden onset of pain and moderate free fluid in the pelvis, suggests that the cyst may have ruptured or torsed, leading to the patient's symptoms.
- Other Likely diagnoses
- Degenerating uterine fibroid: The patient's history of uterine fibroids and recent cessation of hormonal therapy could lead to degeneration of a fibroid, causing pain and tenderness. However, the absence of significant uterine tenderness makes this less likely.
- Pelvic inflammatory disease (PID): Although the patient's temperature is normal, PID could still be considered, especially if there's a possibility of recent sexual activity or other risk factors. However, the lack of cervical motion tenderness and adnexal tenderness (other than the ovarian cyst) makes this less likely.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
- Ectopic pregnancy: Despite a negative urine pregnancy test, ectopic pregnancy must be considered in any woman of childbearing age presenting with abdominal pain, especially if there's a possibility of pregnancy. The recent attempt to conceive increases this risk.
- Appendicitis: Although the pain is in the left lower quadrant, appendicitis can occasionally present with pain in this location, especially if the appendix is retrocecal. The absence of rebound tenderness and guarding does not rule out appendicitis.
- Ovarian torsion (also mentioned under the most likely diagnosis, but included here for emphasis): This is a surgical emergency that requires prompt intervention to preserve ovarian function.
- Rare diagnoses
- Endometriosis: While endometriosis can cause chronic pelvic pain, it is less likely to cause acute, severe pain like that described in this scenario, unless there's an endometrioma that has ruptured.
- Adnexal tumor: Although less common, an adnexal tumor (e.g., a cystadenoma or borderline tumor) could present with similar symptoms, especially if it undergoes torsion or rupture. The ultrasound finding of a simple-appearing cyst makes this less likely, but it cannot be entirely ruled out without further evaluation.