A 37-year-old woman comes to the emergency depament with sudden onset of left leg weakness. There is no history of any prior medical illness. On neurological examination, there is no wasting, muscle tone is normal, deep-tendon reflexes are normal, and plantars show 'flexor' response. The patient cannot raise her left leg off the bed while supine and is very worried that something is wrong. MRI brain does not reveal any abnormality. Which of the following is the most likely diagnosis?