## Core Concept
Heparin is a widely used anticoagulant in cardiac surgery, primarily administered to prevent thrombosis during cardiopulmonary bypass. It works by enhancing the activity of antithrombin III, a natural anticoagulant in the blood. This mechanism is crucial for understanding its effects and interactions.

## Why the Correct Answer is Right
The correct statement about heparin involves its mechanism of action and clinical use. Heparin exerts its anticoagulant effect by activating antithrombin III, which then inhibits thrombin (Factor IIa) and other coagulation factors like Factor Xa. This is a critical aspect of its use in cardiac surgery, as it prevents clotting in the bypass circuit.

## Why Each Wrong Option is Incorrect
- **Option A:** If a statement claims heparin is commonly used in cardiac surgery, which is true, then it cannot be the incorrect statement regarding heparin.
- **Option B:** If a statement about heparin's mechanism of action or its effect on coagulation factors is accurate, it would not be the correct answer to "except."
- **Option C:** Similarly, if another statement accurately describes a property or use of heparin, it wouldn't be the incorrect statement.
- **Option D:** This option would represent a statement that does not accurately describe heparin, such as an incorrect claim about its metabolism, reversal agent, or a side effect.

## Clinical Pearl / High-Yield Fact
A key point to remember about heparin is that its effect can be reversed by protamine sulfate, which is particularly important in cardiac surgery where precise control of anticoagulation is critical. Heparin-induced thrombocytopenia (HIT) is a notable side effect, requiring careful monitoring of platelet counts in patients on long-term heparin therapy.

## Correct Answer: D.