**Core Concept**
Edema over the mastoid is a clinical manifestation of a specific condition that involves the area behind the ear. This condition is related to the anatomy of the mastoid process and the nearby structures.

**Why the Correct Answer is Right**
The correct answer is associated with mastoiditis, an infection of the mastoid process, which is a part of the temporal bone. Mastoiditis often occurs as a complication of otitis media, an infection of the middle ear. The infection spreads to the mastoid process, causing inflammation and edema. The edema is a result of increased blood flow and fluid leakage from the affected mastoid cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because edema over the mastoid is not typically seen in conditions affecting the parotid gland, such as parotitis.
**Option B:** This option is incorrect because edema over the mastoid is not a characteristic feature of Bell's palsy, which affects the facial nerve.
**Option C:** This option is incorrect because edema over the mastoid is not typically seen in conditions such as otosclerosis, which affects the middle ear.

**Clinical Pearl / High-Yield Fact**
Remember that mastoiditis is a surgical emergency that requires prompt treatment to prevent complications such as intracranial extension of the infection.

**Correct Answer:** A. Grisel's syndrome