A 44-year-old female presents with a house so cluttered that most rooms are unusable. She cannot discard items including newspapers from 10 years ago, broken appliances, and empty containers. She reports intense distress when she considers discarding anything and says, "I might need it someday." She does not collect items for their monetary value. The clutter has led to a fire code violation. Her mood is euthymic. What is the MOST likely diagnosis?