Unsuspected cystic left upper quadrant mass

A 27-year-old African American female with no significant medical history presented with left upper quadrant pain radiating to the back, which had been worsening over several months. She had an inability to tolerate oral intake, progressive nausea and vomiting. A review of her symptoms revealed palpitations, mild shortness of breath and two episodes of recent bilious vomiting without haematemesis. Physical examination showed diffuse abdominal tenderness on palpation and mild tachycardia. Laboratory evaluation showed no significant abnormalities, including normal pancreatic and liver enzymes. The chest radiograph was normal. A contrast-enhanced CT of the abdomen was performed with a 64-MDCT scanner (Aquilion 64; Toshiba Medical Systems, Nasu, Japan) with the following scanning parameters: rotation time, 0.5 s; beam collimation, 3261.0 mm; section thickness and interval, 5 mm; helical pitch, 27; 120 kV and tube current in the automatic milliampere exposure setting. Coronal reconstructions were obtained at 4 mm thickness and 4 mm intervals. The patient received 100 ml of intravenous (iv) iohexol (Omnipaque 300, Nycomed, Amersham, UK) administered via a mechanical injector at 3.0 ml s with a scanning delay determined by real-time contrast tracking, starting at 70 HU added to unenhanced liver. What are the CT findings? What is the most likely diagnosis?