Brachial Plexus

Just a quick explanation in case it's not clear on the image. I also recommend going through the motions with your hand. It helps! :3

Distal lesions will affect the lumbricals, so the fingers cannot extend.
Proximal lesions will affect the flexors, so the hand cannot make a fist.

A distal lesion of the ulnar nerve will cause the patient to have difficulty uncurling the 4th and 5th digit, but the 2nd and 3rd digit can. A proximal lesion to the medial nerve will cause the patient to have difficulty curling the 2nd and 3rd digit, but the 4th and 5th can. Both will end up looking like the Pope blessing.

A proximal lesion to the ulnar nerve will cause the patient to have difficulty curling the last two digits while a distal lesion to the median nerve will cause the patient difficulty uncurling the 2nd and 3rd digit. Both will end up looking like the OK gesture.