Question category: Neurology
A 28 year old who is 7 months pregnant presents with pain and tingling in her left hand which she notices is worst in the morning. She has also noticed dropping things. On examination there is 4/5 power in the left abductor pollicis brevis and of thumb flexion. There is reduced sensation in the left thumb, index, middle and half of the lateral half of the ring finger. There is nil else of note. What is the most likely structure affected?