A 27-year-old female presents to the ED with paralysis of her left arm. A thorough medical workup (including labs, chemistries, EMG, and neuroimaging) is unremarkable. She denies any recent trauma or injury to the left arm but does report a recent history of miscarriage. A staff member witnesses the patient moving her left extremity when distracted or dressing, and this is corroborated by the medical resident who noted contraction of the tricep muscle while testing bicep strength (co-contraction sign) on exam. The patient requests to be discharged and does not seem interested in receiving medical care or a diagnosis. Which of the following is the most likely diagnosis?
A. Malingering
B. Transient ischemic attack
C. Brachial plexus injury
D. Functional neurological symptom disorder