In osteogenesis imperfecta, which of the following is defective?

Osteogenesis imperfecta (OI) is a group of genetic diseases of collagen in which the bones are formed improperly, making them fragile and prone to breaking.Collagen is a fibrous protein material. It serves as the structural foundation of skin, bone, cailage, and ligaments. In osteogenesis imperfecta, the collagen produced is abnormal and disorganized. This results in a number of abnormalities throughout the body, the most notable being fragile, easily broken bones.There are four forms of OI, Types I through IV. Of these, Type II is the most severe and is usually fatal within a sho time after bih. Types I, III, and IV have some overlapping and some distinctive symptoms, paicularly weak bones.Ref: Ganong&;s review of medical physiology; 24th edition; page no-386