**Core Concept**
Pott's puffy tumor is a rare but serious complication of frontal sinusitis, characterized by subperiosteal frontal bone abscess. It occurs when an infection spreads from the frontal sinus to the subperiosteal space behind the frontal bone, leading to a collection of pus and swelling.

**Why the Correct Answer is Right**
The infection typically arises from a frontal sinusitis that has become secondarily infected, often with Staphylococcus aureus. The infection spreads through the frontal sinus ostium, which is a small opening in the frontal bone that connects the frontal sinus to the nasal cavity. The abscess then forms in the subperiosteal space behind the frontal bone, leading to localized swelling, pain, and potentially life-threatening complications.

**Why Each Wrong Option is Incorrect**
* **Option A:** While subdural empyema is a serious complication of sinusitis, it occurs in the subdural space beneath the dura mater, not in the subperiosteal space behind the frontal bone.
* **Option B:** Epidural abscess is a complication of osteomyelitis or meningitis, but it occurs in the space between the dura mater and the skull, not in the subperiosteal space behind the frontal bone.
* **Option C:** Subcutaneous abscess is a localized collection of pus in the skin and subcutaneous tissue, but it is not a complication of frontal sinusitis.

**Clinical Pearl / High-Yield Fact**
Pott's puffy tumor is a classic example of a "craniofacial" infection, which can spread from the frontal sinus to the surrounding bone and brain tissue, highlighting the importance of early recognition and treatment of frontal sinusitis.

**Correct Answer: C. Subperiosteal frontal bone abscess.**