**Core Concept**
Subdural empyema is a rare but serious intracranial complication that occurs when an infection spreads from the paranasal sinuses or mastoid air cells, bypassing the dura mater, to the subdural space. This condition can lead to increased intracranial pressure, brain herniation, and even death if left untreated.

**Why the Correct Answer is Right**
Boils, or furuncles, are localized skin infections caused by Staphylococcus aureus. Unlike sinusitis, mastoiditis, or otitis media, boils do not have a direct anatomical connection to the intracranial structures and therefore cannot directly lead to subdural empyema. The other options (A, B, and C) all have a plausible pathway for the spread of infection to the subdural space.

**Why Each Wrong Option is Incorrect**
**Option A:** Frontal sinusitis can lead to subdural empyema through direct extension of infection through the frontal bone into the subdural space.
**Option B:** Skull vault osteomyelitis can also lead to subdural empyema through direct extension of infection into the subdural space.
**Option C:** Middle ear disease, particularly mastoiditis, can lead to subdural empyema through the spread of infection through the petrous apex or through the lateral sinus.

**Clinical Pearl / High-Yield Fact**
Subdural empyema is a medical emergency that requires prompt diagnosis and treatment. High index of suspicion is required in patients with signs of increased intracranial pressure, seizures, or altered mental status, especially in those with a history of upper respiratory tract infections or sinusitis.

**✓ Correct Answer: D. Boil over face**