**Fontanelle Closure**

**Core Concept**
Fontanelles are soft, fibrous areas of the skull where the bones have not yet fused together. The timing and sequence of fontanelle closure are critical in pediatric development, as delayed closure can be a sign of underlying conditions.

**Why the Correct Answer is Right**
The posterior fontanelle is the last fontanelle to close, typically by 6-8 weeks of age. This is because the bones of the posterior fontanelle are relatively thin and flexible, allowing for greater mobility during fetal development. The closure of the posterior fontanelle is a result of the gradual fusion of the occipital bones.

**Why Each Wrong Option is Incorrect**
**Option A:** The anterior fontanelle is the first to close, but it can take up to 18-24 months for complete closure. The anterior fontanelle is a larger fontanelle that is located at the top of the head.
**Option B:** The sphenoidal fontanelle is a small fontanelle located between the sphenoid bone and the parietal bone, and it typically closes by 6 weeks of age. However, it is not the last fontanelle to close.
**Option C:** The mastoid fontanelle is not a true fontanelle, but rather a small depression in the mastoid process of the temporal bone. It is not a significant developmental landmark.

**Clinical Pearl / High-Yield Fact**
Delayed closure of the posterior fontanelle can be a sign of conditions such as rickets, hypothyroidism, or Down syndrome.

**Correct Answer: D. Posterior fontanelle.