A female presented with loss of extension of little and ring finger, hypothenar atrophy and metacarpophalangeal joint hyperextension. The nerve injured is?

When the ulnar nerve is injured in the hand, there is a loss of the interossei and lumbricals 3 and 4, and clawing of digits 4 and 5 may become apparent due to an imbalance of the extrinsic and intrinsic muscles. Because the extrinsic extensors of the hand are not opposed by the intrinsic flexors of the hand, the metacarpophalangeal joint hyperextends and is unable to extend the proximal and distal interphalangeal joints. The proximal and distal interphalangeal joints continue to flex because the extrinsic flexors are not opposed by the intrinsic extensors of the distal and proximal interphalangeal joints. The result is extension of the metacarpophalangeal joint and flexion of the proximal and distal interphalangeal joints. Hypothenar atrophy also seen.