A 40-year-old female presents with sudden onset of right-arm weakness. She cannot lift her right arm and reports it "feels dead." Neurological exam reveals inconsistencies: she has full strength with distraction testing, and her arm reflexes are intact. MRI and nerve conduction studies are normal. She recently experienced a significant interpersonal conflict with her mother. She appears unconcerned about the deficit ("la belle indifférence"). What is the MOST likely diagnosis?