A 45 year old woman develops abdominal and pelvic discomfo. Physical examination reveals a large mass in the right lower quadrant, which is surgically resected. The mass consists of a large (25 cm) cystic sac containing thick mucinous fluid within a thin wall. On careful inspection, the pathologist finds an area of increased thickness in the cyst wall, which is sampled for histology. Microscopically, the tumor appears to be composed mostly of a single layer of nonciliated columnar cells arranged in papillary projections. The thickened area, however, displays stratification of epithelial cells, increased cytologic atypia, and high mitotic activity. Neveheless, no stromal invasion is found. Which of the following is the most likely diagnosis?