**Core Concept**
Hydrocephalus is a condition characterized by an accumulation of cerebrospinal fluid (CSF) within the brain, leading to increased intracranial pressure and potentially causing damage to the brain. In infants, hydrocephalus can result from various causes, including congenital malformations, infections, or tumors.

**Why the Correct Answer is Right**
In infants, ultrasound is the preferred imaging modality for diagnosing hydrocephalus due to its non-invasive nature, lack of ionizing radiation, and ability to provide real-time images. Ultrasound can accurately measure the size of the ventricles and detect any abnormalities in CSF flow. The anterior fontanelle, a soft spot on the top of the infant's head, serves as a natural acoustic window, allowing for clear visualization of the intracranial structures.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan is not typically used as the first-line imaging modality in infants due to the risk of radiation exposure and the difficulty in obtaining clear images through the intact fontanelles.
**Option B:** MRI is not the preferred initial imaging modality in infants due to its higher cost, longer scanning time, and potential need for sedation.
**Option C:** X-ray is not suitable for diagnosing hydrocephalus as it does not provide sufficient detail of the intracranial structures.

**Clinical Pearl / High-Yield Fact**
In infants, it is essential to perform a thorough neurological examination, including assessment of cranial sutures and fontanelles, to identify signs of increased intracranial pressure, such as a bulging anterior fontanelle.

**Correct Answer: B. MRI is not the preferred initial imaging modality in infants due to its higher cost, longer scanning time, and potential need for sedation.