**Core Concept**
An acute retropharyngeal abscess is a serious infection located in the retropharyngeal space, which is a potential space in the neck behind the pharynx. This condition often presents with severe throat pain, difficulty swallowing, and can lead to airway compromise if not promptly treated.

**Why the Correct Answer is Right**
The correct answer is D. Caries of cervical spine is usually a common cause, because caries of the cervical spine is not typically associated with the development of acute retropharyngeal abscess. The common causes of acute retropharyngeal abscess include bacterial infections such as Streptococcus and Staphylococcus, particularly in children, and can also result from contiguous spread of infection from the oropharynx or other neck infections. In adults, lymph node enlargement or metastatic disease can also contribute to the development of a retropharyngeal abscess.

**Why Each Wrong Option is Incorrect**
* **Option A:** Dysphagia (difficulty swallowing) is a common symptom of acute retropharyngeal abscess due to the swelling and inflammation in the retropharyngeal space.
* **Option B:** Swelling on the posterior wall of the pharynx is a characteristic feature of acute retropharyngeal abscess, as the infection causes inflammation and edema in the retropharyngeal space.
* **Option C:** Torticollis (twisted neck) can occur in acute retropharyngeal abscess due to the spasm of neck muscles in response to pain and inflammation.

**Clinical Pearl / High-Yield Fact**
The clinical presentation of acute retropharyngeal abscess can be variable, but it often includes neck pain, difficulty swallowing, and fever. Prompt recognition and treatment are essential to prevent airway compromise and other serious complications.

**✓ Correct Answer: D. Caries of cervical spine is usually a common cause**