Which type of osteogenesis imperfecta is also known as imperfecta with blue sclera?
**Core Concept**
Osteogenesis imperfecta (OI) is a genetic disorder characterized by brittle bones, often resulting from mutations in the COL1A1 or COL1A2 genes, which encode the alpha chains of type I collagen. The condition affects the production and structure of collagen, leading to bone fragility and other systemic features.
**Why the Correct Answer is Right**
The correct answer refers to OI type I, also known as "blue sclera" or "imperfecta with blue sclera." This type is characterized by the presence of blue sclera due to the transparency of the sclera, which is a result of the defect in type I collagen production. The blue discoloration occurs because the sclera contains collagen fibers that are visible through the translucent tissue. This type is typically associated with normal or near-normal bone density and is often considered the mildest form of OI.
**Why Each Wrong Option is Incorrect**
* **Option A:** OI type II is a severe form of the disorder, characterized by poor intrauterine growth, fractures at birth, and often lethal. It is not associated with blue sclera.
* **Option B:** OI type III is a progressive form of the disorder, characterized by multiple fractures, blue sclera, and often a characteristic "beading" of the ribs. However, it is not the specific type known as "imperfecta with blue sclera."
* **Option C:** OI type IV is a moderate form of the disorder, characterized by fractures in early childhood, blue sclera, and often a characteristic "skeletal dysplasia." However, it is not the specific type known as "imperfecta with blue sclera."
**Clinical Pearl / High-Yield Fact**
OI type I can be distinguished from other forms of the disorder by the presence of blue sclera, which can be a useful diagnostic clue in clinical practice. Additionally, OI type I often has a relatively good prognosis, with many patients experiencing a relatively normal life span.
**Correct Answer:** B. Osteogenesis imperfecta type III.