**Core Concept**
Hydrocephalus is a condition characterized by an accumulation of cerebrospinal fluid (CSF) in the brain, leading to increased intracranial pressure and potentially causing damage to the brain and surrounding structures. The investigation of choice for hydrocephalus in infants is crucial for accurate diagnosis and management.

**Why the Correct Answer is Right**
The investigation of choice for hydrocephalus in infants is ultrasound, which is a non-invasive and safe imaging modality that can accurately measure the ventricular size and detect any abnormalities in CSF flow. This is particularly useful in infants due to the open fontanelles, which allow for better visualization of the brain. Additionally, ultrasound is widely available, cost-effective, and can be performed at the bedside.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan is not the investigation of choice for hydrocephalus in infants due to radiation exposure, which is a significant concern in pediatric patients. While CT scans can provide detailed images of the brain, they are generally reserved for when ultrasound is inconclusive or when other imaging modalities are not available.

**Option B:** MRI is a sensitive imaging modality for detecting hydrocephalus, but it is not the first-line investigation in infants due to the need for sedation and the longer scanning time. MRI is typically reserved for when more detailed information is required or when other imaging modalities are inconclusive.

**Option C:** X-ray is not a suitable investigation for hydrocephalus as it does not provide sufficient information about the brain and surrounding structures. X-rays are generally used for detecting bone abnormalities and are not a reliable tool for diagnosing hydrocephalus.

**Clinical Pearl / High-Yield Fact**
In infants, ultrasound is the preferred imaging modality for hydrocephalus due to its non-invasive nature, safety, and ability to detect abnormalities in CSF flow. The presence of open fontanelles in infants allows for better visualization of the brain, making ultrasound an essential tool in the diagnosis and management of hydrocephalus.

**Correct Answer: C. X-ray is not a suitable investigation for hydrocephalus as it does not provide sufficient information about the brain and surrounding structures. X-rays are generally used for detecting bone abnormalities and are not a reliable tool for diagnosing hydrocephalus.