**Question:** Hypercoagulability due to defective factor V gene is called:

A. Factor V Leiden
B. Protein C deficiency
C. Protein S deficiency
D. Antithrombin III deficiency

**Core Concept:**
The question is about a genetic disorder that results in an increased risk of blood clot formation due to a defect in the factor V gene. Factor V is an essential protein involved in the coagulation cascade, particularly in the activation of factor VIII and the enhancement of the intrinsic pathway.

**Why the Correct Answer is Right:**
A. Factor V Leiden: This is the correct answer because it refers to a specific mutation in the factor V gene that leads to hypercoagulability. This genetic variant results in a protein with increased activity, causing a hypercoagulable state and an increased risk of blood clot formation.

**Why Each Wrong Option is Incorrect:**
B. Protein C deficiency: Protein C is an anticoagulant protein that plays a crucial role in the regulation of the coagulation cascade. The deficiency of protein C leads to increased blood clot formation, but the question refers to a defect in the factor V gene, which is distinct from this condition.
C. Protein S deficiency: Similarly to option B, protein S is an anticoagulant protein involved in regulating the coagulation cascade. A deficiency of protein S would lead to increased blood clot formation, but the correct answer is a defect in the factor V gene.
D. Antithrombin III deficiency: Antithrombin III is another anticoagulant protein that inhibits thrombin and factor Xa, which are essential components of the coagulation cascade. A deficiency of antithrombin III would lead to increased blood clot formation, but the question refers to a defect in the factor V gene, which is distinct from this condition.

**Clinical Pearl:**
Factor V Leiden is a genetic disorder that results from a mutation in the F5 gene, which codes for the prothrombin protein. This mutation leads to increased activation of factor V, enhancing the intrinsic pathway and increasing the risk of thrombosis. This genetic disorder is a significant risk factor for venous thromboembolism, including deep vein thrombosis and pulmonary embolism. Knowledge of such genetic disorders helps clinicians recognize and manage patients who are at higher risk for these conditions, enabling appropriate preventive measures and early intervention when needed.