**Core Concept**
Spalding's sign refers to the flattening of the skull in infants with hydrocephalus, which is a condition characterized by an accumulation of cerebrospinal fluid (CSF) within the cranial vault. This occurs due to impaired CSF circulation, leading to increased intracranial pressure and subsequent skull deformation.

**Why the Correct Answer is Right**
In infants with hydrocephalus, the increased intracranial pressure causes the bones of the skull to become flattened, resulting in the characteristic Spalding's sign. This occurs because the cranial sutures, which are the fibrous joints between the bones of the skull, are still open in infants. As a result, the bones of the skull can deform and become flattened under the increased pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because Spalding's sign is not caused by a congenital anomaly such as craniosynostosis, which is a premature fusion of the cranial sutures.
* **Option B:** This is incorrect because Spalding's sign is not caused by a decrease in intracranial pressure, but rather an increase due to hydrocephalus.
* **Option C:** This is incorrect because Spalding's sign is not caused by a tumor or other mass lesion within the cranial vault.

**Clinical Pearl / High-Yield Fact**
Spalding's sign is a classic radiographic finding in infants with hydrocephalus, and its presence should raise suspicion for this condition. It is essential to differentiate hydrocephalus from other causes of intracranial pressure, such as craniosynostosis or tumors.

**Correct Answer:** D.