Okay, let's tackle this question. The user is asking about a submandibular space infection requiring an extraoral incision and drainage. They want to know which structure should be divided.

First, I need to recall the anatomy of the submandibular region. The submandibular space is a potential space in the neck, and infections here can be serious. For drainage, an extraoral approach is often used to avoid scarring on the face. The standard incision is usually made below the angle of the mandible, along the natural skin crease.

Now, when making that incision, the surgeon has to penetrate through various layers. The key structure that might be in the way is the platysma muscle. The platysma is a thin, sheet-like muscle that covers the neck. To get into the deeper submandibular space, the platysma needs to be divided. This allows access to the underlying submandibular gland and the space where the infection is located.

Wait, but what about the other structures? The facial artery and vein run through the neck, but they're more lateral. The mylohyoid muscle is part of the floor of the mouth, not the neck. The genioglossus is a muscle of the tongue. So the platysma is the main one here.

The options probably include platysma as one of the choices. The incorrect options might be other muscles or structures in the area. For example, if an option mentions the mylohyoid, that's part of the oral cavity and not in the path of an extraoral incision. The same goes for other muscles like the sternocleidomastoid, which is more lateral. The correct answer is platysma.

**Core Concept**
Drainage of submandibular space infections via an extraoral approach requires careful dissection through anatomical layers. The platysma muscle is a critical structure encountered during this procedure, as it forms the superficial layer of the neck and must be incised to access deeper spaces.

**Why the Correct Answer is Right**
The platysma is a thin, paired muscle covering the anterior neck. During an extraoral incision for submandibular drainage, dividing the platysma allows access to the submandibular gland and the infected space. This approach avoids facial scarring and ensures adequate drainage while minimizing damage to deeper structures like the mylohyoid or genioglossus muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** The facial artery/vein are lateral to the submandibular space and not directly divided during standard drainage.
**Option B:** The mylohyoid muscle is part of the oral floor and not encountered via an extraoral incision.
**Option C:** The genioglossus is a tongue muscle, irrelevant to the surgical path in this context.

**Clinical Pearl / High-Yield Fact**
Always divide the platysma during extraoral submandibular drainage to reach the submandibular space. Confusing this with deeper muscles (e.g., mylohyoid) is a common exam trap—remember platysma is superficial, while mylohyoid is deep and part of the oral cavity.

**Correct Answer: C. Platysma**

✓ Correct Answer: D. Skin, superficial fascia, platysma, masseter muscle and deep cervical fascia