**Core Concept:**
The posterior fontanelle, also known as the "soft spot" on a baby's head, is a temporary opening in the skull that allows for some degree of flexibility and growth during childhood. In humans, the fontanelles close sequentially as the skull bones fuse together.

**Why the Correct Answer is Right:**
The correct answer, **D**, refers to the closure of the posterior fontanelle, which occurs later than the anterior fontanelle. In infants, the anterior fontanelle (also known as the "gap of Vater") closes between the ages of 6-18 months, while the posterior fontanelle (also known as the "gap of Pfeiffer") closes later, typically between 2-4 years of age.

**Why Each Wrong Option is Incorrect:**
A. The closure of the posterior fontanelle is not associated with any specific age range, making it incorrect.
B. This option also refers to the closure of the anterior fontanelle, which occurs at a different age range (6-18 months).
C. The posterior fontanelle does not fuse due to a deficiency of bone growth, making this option incorrect as well.

**Clinical Pearl / High-Yield Fact:**
In cases where the posterior fontanelle closure is delayed, it may indicate abnormalities such as cleft palate, microcephaly, or developmental disorders like Down syndrome. A delayed closure can be a clue for a thorough pediatric evaluation.

**Correct Answer: D**