**Core Concept**
Congenital hip dislocation (CHD) is a condition where the hip joint is not properly formed, leading to the femoral head being displaced from the acetabulum. In neonates, early detection is crucial for effective management. The investigation of choice should ideally be non-invasive, painless, and easily accessible.

**Why the Correct Answer is Right**
The Barlow and Ortolani tests are clinical maneuvers used to assess the hip joint in neonates. However, these tests require expertise and may not be conclusive. Ultrasonography (USG) has emerged as the investigation of choice due to its ability to visualize the hip joint in real-time, detect subtle abnormalities, and guide further management. The ultrasound scan typically involves a transabdominal or transperineal approach, and the images are obtained with the baby in the supine position.

**Why Each Wrong Option is Incorrect**
**Option A:** Plain radiography is not the investigation of choice for diagnosing congenital hip dislocation in neonates. Although X-rays can be useful in older children and adults, they are not suitable for neonates due to the high radiation dose and the soft tissue masking the hip joint.

**Option B:** CT scan is not necessary for diagnosing congenital hip dislocation in neonates. CT scans involve higher radiation doses and are generally avoided in infants due to the risk of radiation-induced malignancy.

**Option C:** MRI is not typically used as the first-line investigation for congenital hip dislocation in neonates. Although MRI provides excellent soft tissue detail, it is generally not necessary for this condition and is more expensive than ultrasound.

**Clinical Pearl / High-Yield Fact**
The key to diagnosing congenital hip dislocation in neonates lies in performing a thorough clinical examination, including the Barlow and Ortolani tests, and supplementing it with ultrasonography to confirm the diagnosis.

**Correct Answer:** C. Ultrasonography.