**Core Concept**
Activated protein C (APC) resistance is a condition where the body's ability to regulate blood clotting is impaired. This occurs due to a mutation in the factor V gene, leading to the production of factor V Leiden, which is resistant to inactivation by activated protein C.

**Why the Correct Answer is Right**
Activated protein C inactivates factor V and factor VIII to regulate blood clotting. In APC resistance, the factor V Leiden mutation prevents activated protein C from inactivating factor V, leading to excessive clot formation. This is a key mechanism underlying the increased risk of thrombosis in individuals with APC resistance.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because APC resistance is primarily caused by the factor V Leiden mutation, not by the inactivation of other proteins.
**Option B:** This option is incorrect because APC resistance is not directly related to the inactivation of factor VIII.
**Option C:** This option is incorrect because the correct mechanism of APC resistance involves the factor V Leiden mutation, not the inactivation of other proteins.

**Clinical Pearl / High-Yield Fact**
Factor V Leiden is a common genetic mutation that increases the risk of deep vein thrombosis (DVT) and pulmonary embolism (PE). It is essential to screen for factor V Leiden in individuals with a history of recurrent DVT or PE.

**Correct Answer: A. Factor V Leiden mutation**