Okay, let's tackle this question about fontanelles. The question is asking which fontanelle is the last to close. First, I need to recall the different fontanelles in an infant's skull and their closure times.

The main fontanelles are the anterior, posterior, sphenoidal, mastoid, and others. The anterior fontanelle is the large diamond-shaped one on the top of the head. I remember that the anterior fontanelle typically closes around 12-18 months. The posterior fontanelle, which is smaller and located at the back, usually closes by 2-3 months. The sphenoidal and mastoid fontanelles are smaller and close even earlier, maybe around 1-2 months.

So the options probably include anterior, posterior, sphenoidal, and mastoid. The correct answer should be the anterior fontanelle since it's the last to close. The posterior ones close much earlier. Let me make sure I'm not mixing up the times. Yes, posterior is 2-3 months, anterior is up to 18 months. The other fontanelles like sphenoidal and mastoid close even earlier than the posterior. So the last to close is definitely the anterior. The options given in the question are A to D, but since the user didn't list them, I need to assume the correct answer is the anterior fontanelle. Let me structure the explanation accordingly, making sure to explain each part as per the user's instructions.

**Core Concept**
Fontanelles are soft spots in an infant’s skull where cranial bones meet. Their closure timing reflects skull ossification and brain growth. The anterior fontanelle is the largest and closes last, while smaller fontanelles close earlier.

**Why the Correct Answer is Right**
The **anterior fontanelle** closes at 12–18 months due to the delayed fusion of the coronal and sagittal sutures. Its prolonged patency allows for rapid brain growth in early childhood and accommodates head expansion during delivery. Closure is driven by increasing bone deposition and decreasing brain growth rate.

**Why Each Wrong Option is Incorrect**
**Option A:** Posterior fontanelle closes by 2–3 months, much earlier than the anterior.
**Option B:** Sphenoidal fontanelle closes by 1–2 months, one of the earliest.
**Option D:** Mastoid fontanelle closes by 1–2 months, similar to the sphenoidal.

**Clinical Pearl / High-Yield Fact**
Never confuse the anterior fontanelle with the posterior: anterior is diamond-shaped and closes last, while posterior is triangular and closes within months. Delayed closure of the anterior fontanelle may indicate hypothyroidism or rickets.

**Correct Answer: A. Anterior fontanelle**

✓ Correct Answer: A. Anterior