**Core Concept**
Congenital scoliosis is a type of scoliosis that results from vertebral anomalies present at birth. The progression of congenital scoliosis depends on the type of vertebral anomaly, which can be classified into different subtypes based on the level of involvement, such as hemivertebrae, unsegmented bars, and block vertebrae.

**Why the Correct Answer is Right**
The progression of congenital scoliosis is least likely in block vertebrae because they are typically stable and do not contribute to the progression of the curvature. Block vertebrae are characterized by the fusion of two or more adjacent vertebrae, resulting in a solid, non-segmented vertebral body. This stability prevents the further progression of the curvature, making block vertebrae a less likely cause of congenital scoliosis progression.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hemivertebrae are a common cause of congenital scoliosis and are associated with a high risk of progression due to the presence of an abnormal, unilateral vertebral body growth.
* **Option B:** Unsegmented bars are a type of vertebral anomaly characterized by a failure of segmentation between two adjacent vertebrae, leading to a bar-like structure that can contribute to the progression of scoliosis.
* **Option D:** Wedge vertebrae, which are characterized by an abnormal, wedge-shaped vertebral body, can also contribute to the progression of congenital scoliosis due to the uneven growth and development of the spine.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the progression of congenital scoliosis is influenced by the type and severity of vertebral anomalies, and a thorough understanding of the underlying anatomy and pathophysiology is crucial for accurate diagnosis and treatment planning.

**Correct Answer:** C.