**Core Concept**
The sutures and fontanelles of the skull are a critical aspect of pediatric neurology, allowing for growth and development of the brain. The timing of fontanelle closure is a key milestone in pediatric assessment.

**Why the Correct Answer is Right**
The posterior fontanelle is the last to close due to the delayed closure of the lambdoid suture. This is because the lambdoid suture is the last to ossify, allowing for additional growth of the occipital bone. The posterior fontanelle typically closes by 2-3 years of age, making it the last to close among the fontanelles.

**Why Each Wrong Option is Incorrect**
* **Option A:** The anterior fontanelle is the first to close, typically within the first 18 months of life. It closes due to the rapid growth of the frontal bone.
* **Option B:** The sphenoidal fontanelle is not a commonly referred fontanelle, and its closure is not a standard milestone in pediatric assessment.
* **Option D:** The mastoid fontanelle is not a fontanelle in the classical sense, but rather a depression in the mastoid process of the temporal bone.

**Clinical Pearl / High-Yield Fact**
It's essential to note that delayed closure of fontanelles can be an indicator of underlying conditions such as hydrocephalus, rickets, or fibrous dysplasia. A thorough medical history and physical examination are necessary to determine the cause of delayed closure.

**Correct Answer:** .