**Core Concept**
Hydrocephalus in infants is a condition characterized by an accumulation of cerebrospinal fluid (CSF) within the brain, leading to increased intracranial pressure and potential damage to the developing brain. Accurate diagnosis is crucial for timely intervention.

**Why the Correct Answer is Right**
Ultrasound (USG) cranium is the preferred imaging modality for diagnosing hydrocephalus in infants due to its non-invasive nature, lack of ionizing radiation, and ability to provide real-time images. This is particularly useful in neonates, where the open fontanelles allow for unobstructed transmission of sound waves, enabling clear visualization of the ventricles and surrounding brain structures. Additionally, USG cranium can detect subtle changes in ventricular size and shape, making it an ideal tool for monitoring hydrocephalus progression.

**Why Each Wrong Option is Incorrect**
**Option B:** CT scan head, while useful for detecting calcifications and hemorrhages, is not the preferred choice for diagnosing hydrocephalus in infants due to radiation exposure concerns.
**Option C:** Encephalogram, a procedure involving the insertion of a catheter into the ventricular system, is invasive and carries risks of infection and bleeding, making it less desirable for initial diagnosis.
**Option D:** Lumbar-puncture, although useful for assessing CSF pressure and composition, does not provide direct visualization of the ventricles and is not a suitable primary diagnostic tool for hydrocephalus.

**Clinical Pearl / High-Yield Fact**
In infants, the cranial sutures are not yet fused, allowing for the transmission of sound waves and enabling accurate diagnosis of hydrocephalus via ultrasound. This characteristic is crucial for clinicians to recognize and utilize this imaging modality effectively.

**✓ Correct Answer: A. USG cranium**