Review Question - QID 214579

A 52-year-old woman presents to her psychiatrist for follow-up evaluation 2 weeks after starting fluoxetine. She was diagnosed with major depressive disorder during her previous visit and says that her symptoms have not improved at all despite taking medication. Her husband is concerned that she may not be taking the pills, but the patient states that she has been very compliant with the medication. Her past medical history is significant for rheumatoid arthritis, and her family history is significant for thyroid problems. She denies chest pain, shortness of breath, feeling cold, or weight gain. Social history reveals that up until 5 years ago she felt energetic and was very successful in her career, but she was fired last year after delivering increasingly lackluster results. Which of the following is the most likely cause of this patient's lack of improvement?