**Core Concept**
Gingival enlargement is a pathological condition characterized by the hypertrophy and hyperplasia of the gingival tissues, leading to an increase in the size and volume of the gingiva. This condition can be caused by various factors, including local irritants, hormonal changes, and certain medications.

**Why the Correct Answer is Right**
The correct answer is related to the use of **tetracycline**, a broad-spectrum antibiotic that can cause gingival enlargement as a side effect. Tetracycline can bind to calcium ions in the gingival tissues, leading to the inhibition of collagen synthesis and the accumulation of collagen in the gingiva, resulting in its enlargement.

**Option A:** This option is incorrect because **amoxicillin**, a penicillin antibiotic, is not typically associated with gingival enlargement.
**Option B:** This option is incorrect because **metronidazole**, an antibiotic used to treat anaerobic infections, does not commonly cause gingival enlargement.
**Option C:** This option is incorrect because **sodium fluoride**, a medication used to prevent tooth decay, is not associated with gingival enlargement.

**Clinical Pearl / High-Yield Fact**
It's essential to note that gingival enlargement can be reversible with the discontinuation of the offending medication and proper oral hygiene practices.

**Correct Answer:** C. Sodium fluoride