Radiographic studies of a 2 year old child brought to an emergency room reveal a new fracture of the humerus and evidence of multiple old fractures in ribs and long bones of the extremities. A social worker wants to initiate prosecution of the parents for child abuse, but an ale emergency room physician notices that, despite the broken arm, the toddler shows minimal bruising. A very careful, directed, physical examination reveals that the toddler has “peculiar teeth,” a blue tinge to the sclera, and unusually mobile joints. The disease that the physician suspects the child has is characterized by an abnormality of which of the following biochemical functions?

**Core Concept**
The underlying biochemical abnormality in the suspected disease leads to a defect in the production of a crucial enzyme, resulting in a specific clinical presentation.

**Why the Correct Answer is Right**
The physician suspects the child has osteogenesis imperfecta (OI), a genetic disorder characterized by brittle bones, blue sclera, and loose joints. This condition is caused by a deficiency in the production of type I collagen, which is essential for bone formation. The defect is due to mutations in the COL1A1 or COL1A2 genes, which code for the pro-alpha1(I) and pro-alpha2(I) chains of type I collagen, respectively. These mutations lead to an abnormality in the biochemical function of collagen synthesis, resulting in the characteristic features of OI.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not relate to the biochemical abnormality in osteogenesis imperfecta. Glycogen storage diseases are a group of disorders caused by defects in glycogen synthesis or breakdown.

**Option B:** This option is incorrect as it refers to a different biochemical abnormality. Sickle cell disease is caused by a mutation in the HBB gene that codes for hemoglobin subunit beta, leading to abnormal hemoglobin production.

**Option C:** This option is incorrect as it is unrelated to the biochemical abnormality in osteogenesis imperfecta. Cystinuria is a genetic disorder caused by a defect in the transport of cystine and other dibasic amino acids in the kidneys.

**Option D:** This option is incorrect as it refers to a different biochemical abnormality. Phenylketonuria (PKU) is caused by a deficiency in the enzyme phenylalanine hydroxylase, which is essential for the metabolism of phenylalanine.

**Clinical Pearl / High-Yield Fact**
Osteogenesis imperfecta is a genetic disorder with an autosomal dominant pattern of inheritance, meaning that a single copy of the mutated gene is sufficient to cause the condition. The condition is characterized by a range of clinical features, including blue sclera, loose joints, and a history of fractures.

**Correct Answer: D. Phenylalanine metabolism**

✓ Correct Answer: A. Collagen type I synthesis