**Core Concept**
Keloids are a type of raised, thickened scar tissue that forms after skin injury, such as cuts, burns, or surgical incisions. They are characterized by an overgrowth of collagen, leading to a firm, rubbery, or firm and fixed nodule. The exact cause of keloid formation is still not fully understood, but it is believed to involve an imbalance in the healing process and an abnormal response to injury.

**Why the Correct Answer is Right**
Keloids are caused by an overproduction of collagen, which is produced by an abnormal response to injury. This leads to an accumulation of collagen fibers, resulting in a raised, thickened scar tissue. The exact mechanism behind keloid formation is not fully understood, but it is believed to involve an imbalance in the healing process, with an overproduction of collagen and an abnormal inflammatory response. The genetic predisposition, hormonal changes, and skin tension also play a role in the development of keloids.

**Why Each Wrong Option is Incorrect**
**Option A:** Keloids are caused by a deficiency in vitamin C.
* Incorrect because keloids are not caused by a deficiency in vitamin C, but rather by an abnormal response to injury and an overproduction of collagen.

**Option B:** Keloids are not painful.
* Incorrect because keloids can be painful, especially if they are large or if they become inflamed.

**Option C:** Keloids are a type of flat scar tissue.
* Incorrect because keloids are a type of raised, thickened scar tissue.

**Clinical Pearl / High-Yield Fact**
To prevent keloid formation, it is essential to keep the wound clean and moist, apply topical silicone gel or pressure to reduce skin tension, and avoid picking or scratching the wound.

**Correct Answer: C. Keloids are a type of raised, thickened scar tissue.**