Final answer:
The most likely condition suspected is an Ovarian cyst, which is a common and usually harmless disorder of the ovaries. Treatment focuses on fertility preservation and may include surgery if cysts are large or painful. Distinguishing between benign and malignant conditions is vital to proper management.
Step-by-step explanation:
Given the presentation of a teenage girl with a large adnexal mass and some weight gain, the most likely condition suspected is Ovarian cyst. Ovarian cysts are common disorders of the ovaries, which can often resolve on their own without treatment. However, in cases where ovarian cysts are very large or painful, surgical intervention may be necessary. When considering treatment for a teenage girl, fertility preservation is an important factor, as some treatments can impact future fertility. It is crucial to distinguish between benign ovarian cysts and other conditions such as ovarian cancer, endometriosis, or uterine fibroids to provide appropriate management. Other important aspects of treatment could involve hormone therapy, especially if the cyst is related to an underlying hormonal disorder like polycystic ovary syndrome (PCOS), which can be associated with weight gain, acne, and excess facial hair.