**Core Concept**
The knee joint is a complex hinge joint stabilized by ligaments, tendons, and bony structures. When the knee is extended from a flexed position, the ligaments and joint capsule are subjected to varying degrees of tension and stress.

**Why the Correct Answer is Right**
When the knee is fully extended from a 90-degree flexed position, the ligaments and joint capsule are stretched, and the menisci are subjected to compression. This movement can lead to a phenomenon known as _knee joint cavitation_, where the pressure within the joint capsule increases, potentially causing a loud popping sound due to the sudden release of trapped gas bubbles in the synovial fluid. This is a common occurrence in healthy individuals and is often referred to as a "knee crack" or "pop."

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the correct mechanism or consequence of knee extension from a flexed position. Knee joint cavitation is a well-documented phenomenon that occurs during this movement.
* **Option B:** This option is incorrect because it does not accurately describe the expected outcome of knee extension from a flexed position. While knee pain or discomfort may occur due to overstretching of ligaments or tendons, it is not the primary consequence of this movement.
* **Option C:** This option is incorrect because it does not accurately describe the expected outcome of knee extension from a flexed position. While knee instability may occur due to ligamentous injury, it is not the primary consequence of this movement in a healthy individual.

**Clinical Pearl / High-Yield Fact**
Knee joint cavitation is a common occurrence in healthy individuals, particularly during activities that involve rapid knee extension, such as jumping or squatting. However, repeated or forceful knee joint cavitation may lead to joint instability or pain in some individuals.

**Correct Answer:** C.