Final answer:
In a 45-year-old female with a long history of oral contraceptive use and RUQ pain with a circumscribed mass on ultrasound, the most likely diagnoses are Hepatic adenoma or Focal nodular hyperplasia.
Step-by-step explanation:
The subject of this question falls into Medicine, specifically dealing with conditions affecting the liver and biliary tree. Given the information provided - a long history of oral contraceptive pill (OCP) use, right upper quadrant (RUQ) pain, and a circumscribed mass on ultrasound in a 45-year-old female - the differential diagnoses include Hepatic adenoma, Gallstones, Liver abscess, and Focal nodular hyperplasia (FNH).
Considering the patient's history and imaging findings, Hepatic adenoma and FNH are more likely connected to OCP use and appear as circumscribed lesions on imaging, whereas gallstones typically lead to biliary obstruction and liver abscess presents with systemically unwell symptoms and signs of infection.