**Core Concept**
Posterior iliac horns are a radiological feature associated with a specific genetic disorder. They are bony projections that extend from the posterior aspect of the iliac bones, visible on plain radiographs or CT scans.

**Why the Correct Answer is Right**
The correct answer is associated with **Familial Adenomatous Polyposis (FAP)**, a genetic disorder characterized by the development of hundreds to thousands of polyps in the colon and rectum. The polyps are typically benign but have a high risk of malignant transformation. The genetic defect behind FAP leads to mutations in the **adenomatous polyposis coli (APC) gene**, which is a tumor suppressor gene. The mutation causes uncontrolled cell growth and leads to the formation of polyps. The formation of posterior iliac horns is a result of the abnormal growth and development of the iliac bones due to the genetic mutation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Osteogenesis imperfecta is a genetic disorder characterized by brittle bones and blue sclerae, but it does not typically present with posterior iliac horns.
* **Option B:** Multiple myeloma is a type of cancer that affects plasma cells in the bone marrow, but it does not typically cause the formation of posterior iliac horns.
* **Option C:** Osteomalacia is a condition characterized by softening of the bones due to vitamin D deficiency, but it does not typically present with posterior iliac horns.

**Clinical Pearl / High-Yield Fact**
FAP is an autosomal dominant disorder, meaning that a single copy of the mutated gene is sufficient to cause the condition. It is essential to screen for FAP in patients with a family history of the disease, as early diagnosis and prophylactic colectomy can prevent the development of colorectal cancer.

**Correct Answer: D. Familial Adenomatous Polyposis (FAP)**