**Core Concept**
The **masticatory space** is an anatomical region that contains the muscles of mastication, and odontogenic infections can spread to this space, leading to severe complications. Infections in this space can be life-threatening due to the potential for airway compromise. The **medial pterygoid muscle** plays a key role in the masticatory space.

**Why the Correct Answer is Right**
The correct answer is related to the **trismus**, which is the most definite clinical sign indicating odontogenic infection in the masticatory space. Trismus occurs due to the inflammation and swelling of the medial pterygoid muscle, leading to limited mouth opening. This is a result of the infection spreading to the masticatory space, causing pain and stiffness in the muscles of mastication.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not specific to the masticatory space.
**Option B:** Incorrect as it is not a direct indicator of infection in the masticatory space.
**Option D:** Incorrect because, although it may be associated with infection, it is not the most definite clinical sign.

**Clinical Pearl / High-Yield Fact**
Trismus is a critical clinical sign that indicates the spread of odontogenic infection to the masticatory space, and prompt treatment is necessary to prevent further complications.

**Correct Answer:** D. Trismus.