**Core Concept**
The patient is presenting with signs and symptoms of a chronic, granulating odontogenic infection, likely a dental abscess or periapical granuloma, which has progressed to an actinomycotic infection characterized by the presence of **Sulphur granules**. The gram stain of the exudate is suggestive of actinomycosis.

**Why the Correct Answer is Right**
The presence of Sulphur granules and the gram stain findings are indicative of actinomycosis, a chronic bacterial infection caused by **Actinomyces israelii**. The infection is characterized by the formation of granules, which are actually colonies of bacteria. The treatment of choice for actinomycosis is a prolonged course of antibiotics, specifically **penicillin**. Penicillin acts by inhibiting cell wall synthesis, thereby preventing the growth and proliferation of the bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** Metronidazole is used to treat anaerobic bacterial infections, but it is not the first-line treatment for actinomycosis.
**Option B:** Amoxicillin is a broad-spectrum antibiotic, but it is not effective against Actinomyces israelii.
**Option C:** Clindamycin is used to treat various bacterial infections, but it is not the preferred treatment for actinomycosis.

**Clinical Pearl / High-Yield Fact**
Actinomycosis is often misdiagnosed as a malignant tumor due to its slow growth and chronic nature. However, the presence of Sulphur granules in the pus is a key diagnostic feature.

**Correct Answer:** C. Penicillin.