A 26-year-old female has cyclic pelvic pain with significant dysmenorrhea. She has a palpable adnexal mass and a negative serum pregnancy test. What is the most likely diagnosis?

The presence of cyclic pelvic pain alongside significant dysmenorrhea, coupled with a palpable adnexal mass and a negative serum pregnancy test, points towards endometriosis as the most likely diagnosis.

Endometriosis is characterized by the presence of endometrial-like tissue outside the uterus, causing inflammation and pain that often correlates with the menstrual cycle. The cyclic nature of the pain indicates that it is affected by hormonal changes, which aligns with endometriosis' response to menstrual cycles. The palpable adnexal mass may represent an endometrioma, which is a type of cyst formed when endometrial tissue grows on the ovaries.

Other conditions listed may present with pelvic pain and adnexal masses but have differing underlying mechanisms or clinical presentations. For instance, functional ovarian cysts could also present with a palpable mass but are typically not associated with significant dysmenorrhea or the cyclic pattern seen in endometriosis. Ovarian cancer is less likely in this age group, especially given the patient's negative pregnancy test. Pelvic inflammatory disease would generally present with additional symptoms, such as fever or abnormal vaginal discharge, and is more likely to be bilateral without the characteristic cyclicity of pain.

Thus, the combination of cyclical symptoms, dys