## **Core Concept**
Ultrasound (USG) is a valuable tool for detecting congenital anomalies during pregnancy. It can identify a wide range of structural abnormalities in the fetus, including those related to the central nervous system, cardiovascular system, and musculoskeletal system. The detection of congenital anomalies by USG is crucial for prenatal counseling and management.

## **Why the Correct Answer is Right**
Anencephaly, a condition characterized by the absence of a major portion of the brain and skull, can be easily detected by USG. Similarly, conditions like spina bifida, which involves an incomplete closing of the backbone and membranes around the spinal cord, and achondroplasia, a form of short-limbed dwarfism, can also be identified through ultrasound imaging. However, certain conditions might not be directly detectable or might be challenging to diagnose prenatally.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Anencephaly can indeed be identified by USG as it involves a significant structural anomaly that can be visualized during the scan.
- **Option B:** Spina bifida is another condition that can be detected through USG, particularly with the use of advanced ultrasound techniques.
- **Option C:** Achondroplasia, being a form of skeletal dysplasia, can also be identified prenatally through ultrasound by observing the characteristic shortening of the limbs.

## **Why Option D is Correct (Implicit)**
- **Option D:** Polydactyly, which is the condition of having extra fingers or toes, can indeed be challenging to diagnose prenatally with certainty using USG, especially if it's a minor isolated finding. However, major structural anomalies like those listed in the other options are generally more straightforward to detect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while USG is a powerful tool for detecting congenital anomalies, its accuracy can depend on the gestational age, the skill of the operator, and the type of anomaly. Certain conditions might only be definitively diagnosed postnatally.

## **Correct Answer:** D. Polydactyly.