**Core Concept**
Edema over the mastoid is a clinical manifestation associated with the spread of infection from the middle ear to the mastoid air cells. This condition is known as mastoiditis, which can lead to serious complications if left untreated.

**Why the Correct Answer is Right**
The spread of infection from the middle ear to the mastoid air cells occurs through the mastoid antrum, a small cavity located behind the ear. The infection can cause inflammation and edema in the surrounding tissues, leading to the characteristic edema over the mastoid. Mastoiditis is often a complication of acute otitis media, which is an infection of the middle ear.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because edema over the mastoid is not typically associated with a condition affecting the temporomandibular joint (TMJ). While TMJ disorders can cause pain and inflammation in the jaw and face, they are not typically characterized by edema over the mastoid.

**Option B:** This option is incorrect because a parotid gland abscess is a separate condition that affects the parotid gland, which is located in front of the ear. While a parotid gland abscess can cause swelling and pain in the facial region, it is not typically associated with edema over the mastoid.

**Option C:** This option is incorrect because a cervical lymphadenitis is an inflammation of the lymph nodes in the neck, which is not typically associated with edema over the mastoid. While cervical lymphadenitis can cause swelling and pain in the neck, it is not typically characterized by edema over the mastoid.

**Clinical Pearl / High-Yield Fact**
Mastoiditis is a serious condition that requires prompt medical attention. The classic triad of symptoms for mastoiditis includes ear pain, fever, and edema over the mastoid. Early recognition and treatment of mastoiditis are crucial to prevent complications such as intracranial spread of infection and hearing loss.

**Correct Answer:** B. Parotid gland abscess.