Froment's sign is positive in cases of weakness of ?

Ans. is 'a' i.e., Thumb adduction Normally when a person is asked to grasp a book between the thumb and index finger, he will grasp the book firmly with thumb extended, taking full advantage of the adductor pollicis and the first dorsal interosseous muscles. If the ulnar nerve is injured the adductor pollicis will be paralysed and the patient will hold the book by using the flexor pollicislongus (supplied by median nerve) producing flexion at the interphalangeal joint. This becomes more pronounced if the examiner tries to pull the book out while the patient tries to hold it. This sign is known as `Froment's sign' or the 'book test'.