**Core Concept**
Tetracycline is a broad-spectrum antibiotic that can cause tooth discoloration, particularly in children whose teeth are still developing. This is due to the binding of tetracycline to calcium ions in the teeth, leading to abnormal pigmentation.

**Why the Correct Answer is Right**
Tetracycline binds to the calcium ions in the teeth, causing a permanent discoloration that can range from yellow to brown. This binding occurs during the formation of the teeth, which is why children are more susceptible to this side effect. The discoloration can be more pronounced in the primary teeth and may not be visible until the permanent teeth erupt.

**Why Each Wrong Option is Incorrect**
* **Option A:** Discoloration of the nails is not a known side effect of tetracycline.
* **Option B:** While tetracycline can cause photosensitivity, it does not cause noticeable discoloration of the skin.
* **Option D:** Tetracycline does not cause discoloration of the hair.

**Clinical Pearl / High-Yield Fact**
Children under the age of 8 should not receive tetracycline due to the risk of tooth discoloration.

**Correct Answer:** C.