In a patient presenting with syncope and irregular menstrual cycles, what is a likely diagnosis if pelvic pain and a left adnexal mass are present?

The presence of syncope, irregular menstrual cycles, pelvic pain, and a left adnexal mass strongly suggests an ectopic pregnancy. Ectopic pregnancy occurs when a fertilized egg implants outside the uterine cavity, commonly in the fallopian tube. This condition can lead to internal bleeding and may explain the symptoms of syncope due to potential blood loss or hemodynamic instability.

Additionally, the irregular menstrual cycles can indicate disrupted hormonal patterns associated with pregnancy and ectopic implantation. The left adnexal mass is significant, as it can represent an enlarged fallopian tube containing the ectopic pregnancy.

In contrast, while a ruptured ovarian cyst and placenta abruptio might also involve pelvic pain, they typically would not explain the combination of syncope and an adnexal mass as seen in this scenario. Pelvic inflammatory disease could cause pelvic pain and irregular menstrual cycles but would not typically present with a discrete adnexal mass and is less likely to cause syncope unless there are complications like abscess formation or severe inflammation leading to systemic effects.