## **Core Concept**
Hydrocephalus is a condition characterized by an accumulation of cerebrospinal fluid (CSF) within the brain, leading to ventricular enlargement. Antenatal detection of hydrocephalus is crucial for early intervention and management. Ultrasound is a primary imaging modality used for antenatal assessment due to its safety and effectiveness.

## **Why the Correct Answer is Right**
The correct answer, **D. Ultrasound**, is the most appropriate choice for antenatal detection of hydrocephalus. Ultrasound allows for the measurement of ventricular size and the assessment of other signs suggestive of hydrocephalus, such as the "banana sign" or "lemon sign." It is widely available, non-invasive, and does not expose the fetus to ionizing radiation, making it the preferred initial imaging technique during pregnancy.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While MRI provides detailed images and is useful in the evaluation of hydrocephalus postnatally or in specific cases antenatally, it is not the first-line imaging modality for antenatal detection due to higher costs, limited availability, and concerns about fetal exposure to magnetic fields.
- **Option B:** X-ray involves ionizing radiation and is not used for the antenatal assessment of hydrocephalus due to potential risks to the fetus.
- **Option C:** CT scans also involve ionizing radiation and are generally avoided in pregnancy unless absolutely necessary. They are not the preferred method for antenatal evaluation of hydrocephalus.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the antenatal ultrasound diagnosis of hydrocephalus often relies on indirect signs, as direct measurement of intracranial pressure is not feasible. The "banana sign," which refers to the abnormal curvature of the cerebellum, and the "lemon sign," which refers to the scalloping of the frontal bones of the skull, are important indicators.

## **Correct Answer: D. Ultrasound**