You are answering a USMLE-style medical question. Question: A 23-year-old woman presents to her psychiatrist concerned about her mood. She has felt tired and unwilling to engage in any activities lately. She states that her limbs feel heavy all the time and that completing any activity takes tremendous effort. She no longer finds any happiness in activities that she previously enjoyed. She also states that she really struggles to sleep and at times can't sleep for several days. The patient is started on appropriate first-line therapy and sent home. She returns 1 week later stating that her symptoms have not improved. She is requesting help as her performance at work and school is suffering. Which of the following is the best next step in management? Options: A) Add lithium to treatment regimen B) Change treatment to lithium C) Continue current therapy D) Electroconvulsive therapy Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.