A 30 yrs nulliparous patient with primary infeility presents with a pelvic mass, the CA - 125 = 90 U/L. Most probable cause is?

CA 125 elevated in 80% of patients with advanced ovarian cancer may also be elevated in pancreatic, gastric, colonic, and breast carcinoma; in non-malignant conditions such as benign cysts, endometriosis, pelvic inflammatory disease, and ascites; and in menstruation and pregnancy. Fuher, it may be normal in 50% of early stage ovarian cancer Since the patient here is young and has presented with infeility and the value of CA 125 is not exceptionally high, the chances of endometrioma are more