## **Core Concept**
The diagnostic evaluation of hydrocephalus, particularly in a one-month-old child, involves imaging techniques to assess the ventricular system and cerebrospinal fluid (CSF) dynamics. Hydrocephalus is characterized by an accumulation of CSF within the brain, leading to ventricular enlargement. The investigation of choice should be safe, non-invasive, and capable of providing detailed images of the brain's ventricular system.

## **Why the Correct Answer is Right**
Ultrasonography, specifically **transfontanelle ultrasound**, is the correct answer because it is a non-invasive, bedside procedure that can be safely performed on infants, including those as young as one month old. It allows for the assessment of ventricular size and morphology, which are critical in diagnosing hydrocephalus. This method does not expose the child to radiation, making it a preferred initial imaging technique in this age group.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **CT scan** can diagnose hydrocephalus by showing ventricular enlargement, it involves radiation, which is a significant concern in infants due to their sensitivity to radiation and the potential long-term risks of radiation exposure.
- **Option B:** **MRI** is highly sensitive for diagnosing hydrocephalus and provides detailed images of brain structures. However, it may require sedation in young children to prevent movement artifacts, making it less immediately accessible than ultrasonography for initial evaluation.
- **Option D:** **X-ray** is not suitable for diagnosing hydrocephalus as it does not provide detailed images of the brain's soft tissues or ventricular system.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in infants, **transfontanelle ultrasound** is often the initial imaging modality of choice for suspected hydrocephalus due to its safety, availability, and ability to assess ventricular size without the need for radiation or sedation.

## **Correct Answer:** .