**Core Concept**
A parapharyngeal abscess is a collection of pus in the parapharyngeal space, which is a potential space located lateral to the pharynx. This condition can arise due to the spread of infection from adjacent structures, such as the tonsils, teeth, or sinuses.

**Why the Correct Answer is Right**
The most common cause of parapharyngeal abscess in adults is the spread of infection from the tonsils, particularly the tonsillar crypt abscess. This occurs when a crypt abscess in the palatine tonsil ruptures through the tonsillar capsule, leading to the spread of infection into the parapharyngeal space. The rich lymphatic network in this area facilitates the spread of infection, making it a common pathway for the development of parapharyngeal abscess.

**Why Each Wrong Option is Incorrect**
* **Option A:** While dental infections can cause parapharyngeal abscess, they are not the most common cause in adults.
* **Option B:** Sinusitis is a potential cause of parapharyngeal abscess, but it is less common than the spread of infection from the tonsils.
* **Option D:** Other causes, such as trauma or neck surgery, can also lead to parapharyngeal abscess, but they are less frequent than the spread of infection from the tonsils.

**Clinical Pearl / High-Yield Fact**
The parapharyngeal space is a potential space, meaning it is not a fixed, defined space, but rather a region that can expand and become distended with inflammation or infection.

**Correct Answer: C. Tonsillar crypt abscess.**