## **Core Concept**
The question tests knowledge of a specific genetic disorder leading to a hypercoagulable state, characterized by a mutation in a factor V gene. This condition results in an abnormal form of factor V, which is a critical protein in the coagulation cascade.

## **Why the Correct Answer is Right**
The correct answer, **C. Factor V Leiden**, is a variant of factor V that is resistant to inactivation by activated protein C (APC). Normally, APC regulates the coagulation cascade by cleaving and inactivating factors Va and VIIIa. The mutation in the factor V gene leads to a substitution at position 506 (R506Q), making factor V resistant to APC-mediated degradation. This results in a prothrombotic state because the coagulation pathway remains active for a longer period.

## **Why Each Wrong Option is Incorrect**
- **Option A: Antithrombin deficiency** refers to a different condition where there is a deficiency in antithrombin, a protein that inhibits the coagulation cascade. This condition also leads to a hypercoagulable state but through a different mechanism.
- **Option B: Protein C deficiency** involves a deficiency in protein C, which is necessary for the inactivation of factors Va and VIIIa. While related to the mechanism of factor V Leiden, it is not caused by a mutation in the factor V gene.
- **Option D: Prothrombin gene mutation** refers to a mutation in the prothrombin gene that can also lead to a hypercoagulable state. However, it is distinct from the factor V Leiden mutation.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Factor V Leiden** is the most common inherited thrombophilia in individuals of European descent. Carriers of the factor V Leiden mutation have an increased risk of venous thromboembolism, particularly in the setting of other risk factors such as surgery, trauma, or oral contraceptive use.

## **Correct Answer: C. Factor V Leiden**