**Core Concept**
Osteogenesis imperfecta (OI) is a genetic disorder characterized by fragile bones, skeletal deformities, and various extrasekeletal features. It is caused by mutations in the genes encoding for type I collagen, a structural protein crucial for bone formation.

**Why the Correct Answer is Right**
Type I collagen, a heterotrimer composed of two alpha-1 chains and one alpha-2 chain, is produced by osteoblasts and is essential for bone matrix formation. Mutations in the COL1A1 and COL1A2 genes, which encode for the alpha-1 and alpha-2 chains of type I collagen, respectively, lead to the production of abnormal collagen molecules. This results in the formation of bones that are fragile, prone to fractures, and have characteristic blue sclerae.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because osteogenesis imperfecta is not caused by mutations in the genes encoding for other types of collagen, such as type II collagen, which is primarily found in cartilage.
**Option B:** This option is incorrect because osteogenesis imperfecta is not caused by a deficiency in vitamin D, which is essential for calcium absorption and bone mineralization.
**Option C:** This option is incorrect because osteogenesis imperfecta is not caused by a defect in the function of osteoblasts, which are the bone-forming cells responsible for producing type I collagen.

**Clinical Pearl / High-Yield Fact**
OI can be classified into four main types, ranging from mild to severe, based on the severity of the bone fragility and the presence of extrasekeletal features.

**Correct Answer: C. Mutations in the genes encoding for type I collagen.**