**Core Concept**
Greenstick and nightstick fractures are types of fractures that occur due to a specific mechanism of injury. Greenstick fractures involve a partial fracture of the bone, where the bone bends and partially breaks, whereas nightstick fractures involve a fracture of a long bone, typically the ulna or radius, resulting from a direct blow to the forearm.

**Why the Correct Answer is Right**
In greenstick and nightstick fractures, the bone is subjected to a bending force, which causes a partial or complete fracture. This type of injury is commonly seen in children, whose bones are more pliable and prone to bending. The mechanism of injury typically involves a fall onto an outstretched hand or a direct blow to the forearm.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a type of fracture, and the question is specifically asking about greenstick and nightstick fractures.

**Option B:** This option is incorrect because a comminuted fracture involves the bone breaking into multiple fragments, which is not the same as a greenstick or nightstick fracture.

**Option C:** This option is incorrect because a stress fracture is a type of overuse injury that occurs due to repetitive stress on the bone, which is different from the mechanism of injury seen in greenstick and nightstick fractures.

**Clinical Pearl / High-Yield Fact**
In children, greenstick fractures are more common due to their flexible bones, and nightstick fractures often occur in the forearm due to a direct blow to the forearm.

**Correct Answer: C. Stress fracture**