**Core Concept**
Actinomycosis is a chronic, granulomatous infection caused by *Actinomyces* species, most commonly *Actinomyces israelii*. It presents as a slow-growing, suppurative infection with a characteristic "cotton wool" appearance on histology and is frequently found in the head and neck region due to the presence of dead tissue and poor drainage.

**Why the Correct Answer is Right**
The mandible is the most common site of actinomycosis due to the presence of bony structures, dental roots, and chronic inflammation from dental infections. *Actinomyces* species are part of the normal oral flora and can invade when the oral mucosa is compromised, such as in periodontal disease or tooth extraction. The infection spreads through the bone, leading to sequestrum formation, abscesses, and a sinus tract, with the mandible being the most frequently affected bone in the skull.

**Why Each Wrong Option is Incorrect**
Option A: Tibia – While tibial infections can occur, they are rare and typically due to trauma or osteomyelitis from other organisms, not actinomycosis.
Option C: Scapula – This area has minimal exposure to oral flora and no known association with actinomycosis.
Option D: Femur – Femoral infections are usually due to hematogenous spread or trauma and are not a typical site for actinomycosis.

**Clinical Pearl / High-Yield Fact**
"Actinomycosis most commonly affects the mandible, especially in patients with poor oral hygiene or dental disease. The classic presentation includes a chronic, painful swelling with a sinus tract and a 'cotton wool' appearance on biopsy."

✓ Correct Answer: B. Mandible