Medical study of skin and its diseases

Answer: Option A

One word substitute is Dermatology.

Dermatology: the branch of medicine concerned with the diagnosis and treatment of skin disorders.
Endocrinology: the branch of physiology and medicine concerned with endocrine glands and hormones.
Gynecologist: a physician or surgeon qualified to practise in gynaecology.
Orthopaedics: the branch of medicine dealing with the correction of deformities of bones or muscles.