**Core Concept**
Acute sinusitis is an inflammatory condition of the paranasal sinuses, often caused by bacterial or viral infections. The complications of acute sinusitis arise from the spread of infection to adjacent structures or the development of secondary consequences such as osteomyelitis or orbital cellulitis.

**Why the Correct Answer is Right**
The correct answer involves the understanding of the anatomical relationships between the paranasal sinuses and surrounding structures. The spread of infection from the sinuses can lead to complications such as orbital cellulitis, where the infection invades the orbital fat and surrounding tissues, causing inflammation and potential vision loss. This is due to the close proximity of the sinuses to the orbit and the potential for direct spread through thin bony walls.

**Why Each Wrong Option is Incorrect**
**Option A:** Meningitis is not a direct complication of acute sinusitis, although it can occur in rare cases, especially in immunocompromised patients. However, it is not the most common or direct complication.

**Option B:** Pneumonia is a separate respiratory infection that can be caused by different pathogens and is not a direct complication of acute sinusitis.

**Option C:** Pharyngitis is an inflammation of the pharynx and can be a symptom of acute sinusitis but is not a direct complication.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the complications of acute sinusitis are often related to the spread of infection to surrounding structures, particularly the orbit and central nervous system. This emphasizes the importance of prompt and effective treatment of acute sinusitis to prevent these serious complications.

**Correct Answer:** C. Pharyngitis.