Review Question - QID 214960

A 37-year-old woman presents to her physician for mood changes and recent falls over the course of 3 months. She feels more irritable with her husband and children and does not find pleasure in creating art anymore. Over the last 2 weeks, she has been sleeping approximately 5 hours a night and had a fall while walking in her home with the lights off. She denies any head trauma or loss of consciousness associated with the fall. She has a medical history of generalized anxiety disorder and hypothyroidism for which she takes escitalopram and levothyroxine. She had a Roux-en-Y gastric bypass 5 years ago. She has been in a monogamous relationship with her husband for the last 15 years and uses a copper intrauterine device. Physical examination is notable for a depressed affect. Pupils are equal, round, and reactive to light and accommodation. She has 4/5 strength in the bilateral lower extremities and brisk patellar reflexes. Which of the following is most likely the cause of this patient's symptoms?