## **Core Concept**
Nail patella syndrome, also known as Fong disease or hereditary onycho-osteodysplasia, is a rare genetic disorder characterized by abnormalities in the development of nails, kneecaps (patellae), elbows, and pelvis. This condition is associated with mutations in the **LMX1B gene**.

## **Why the Correct Answer is Right**
The correct answer involves understanding the classic features of nail patella syndrome. Key characteristics include:
- **Iliac horn**: A bony projection from the iliac wing, which is a common and significant radiographic finding.
- **Absent patella or patellar hypoplasia**: The kneecap may be small, absent, or irregularly shaped.
- **Nail abnormalities**: Patients often have underdeveloped or absent nails, particularly on the thumbs and index fingers.
- **Elbow and knee deformities**: These can include limited extension or flexion due to anatomical abnormalities.

## **Why Each Wrong Option is Incorrect**
- **Option B: Sacral horn**: While iliac horns are characteristic, sacral horns are not typically mentioned as a primary feature of nail patella syndrome.
- **Option D: Knee deformity**: Although knee deformities can occur due to patellar abnormalities, this option is too nonspecific compared to the more specific features listed.
- **Option E: Dislocation of patella**: While dislocation can be a complication due to patellar hypoplasia or malformation, it is not as universally characteristic as absent patella or iliac horn.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **nail patella syndrome** can lead to **renal disease** in a subset of patients, including **Focal Segmental Glomerulosclerosis (FSGS)**, highlighting the importance of long-term follow-up beyond musculoskeletal and dermatological manifestations.

## **Correct Answer:** C.