**Core Concept**
Pauwel's angle is a measure of the angle between the axis of a fracture line and the horizontal plane. It is used to assess the stability of an intertrochanteric hip fracture. An increase in Pauwel's angle indicates a more vertical fracture line, which can compromise the stability of the fracture and make it more challenging to achieve a satisfactory reduction and fixation.

**Why the Correct Answer is Right**
An increase in Pauwel's angle is associated with a higher risk of fracture instability and nonunion. This is because the more vertical fracture line creates a greater moment arm for the forces acting on the fracture, making it more difficult to achieve a stable reduction. In addition, the increased angle can also compromise the ability of the surrounding bone to provide adequate support and stability to the fracture.

**Why Each Wrong Option is Incorrect**
* **Option A:** A decrease in Pauwel's angle would indicate a more horizontal fracture line, which is generally associated with a lower risk of instability and nonunion.
* **Option B:** A change in Pauwel's angle is not directly related to the bone density of the patient. While bone density can affect the risk of fracture and the stability of the fracture, it is not a direct determinant of Pauwel's angle.
* **Option C:** A change in Pauwel's angle is not directly related to the type of fixation used to treat the fracture. While the type of fixation can affect the stability of the fracture, it is not a direct determinant of Pauwel's angle.

**Clinical Pearl / High-Yield Fact**
When assessing an intertrochanteric hip fracture, it is essential to consider the Pauwel's angle as part of the overall evaluation. A more vertical fracture line (increased Pauwel's angle) can increase the risk of fracture instability and nonunion, and may require more aggressive or specialized treatment.

**Correct Answer:** D.