The answer is the first one (a): Onychomalacia.
Explanation for each option:
a. Onychomalacia: This term is derived from "onycho-" meaning nail and "-malacia" meaning softening. Therefore, onychomalacia specifically refers to the abnormal softening of the nail, making it the correct answer.
b. Onychia: This term refers to inflammation of the nail bed. It does not relate to the softening of the nail, so it is incorrect.
c. Onycholysis: This term describes the separation of the nail from the nail bed, not the softening of the nail. Therefore, it is incorrect.
d. Onychopathy: This is a general term for any disease of the nails. While it could encompass a variety of nail conditions, it does not specifically mean softening, so it is incorrect.
e. Onychophagia: This term refers to the habit of nail biting. It does not relate to the softening of the nail, so it is incorrect.
In summary, the correct medical term for abnormal softening of the nail is onychomalacia.