Q-Bank Breakdown: Factor V Leiden — Why Every Answer Choice Matters

You’re doing a Q-bank question, you spot a DVT/PE vignette in a young patient, and your brain instantly jumps to “Factor V Leiden.” That’s good—but Step questions aren’t just asking if you recognize the buzzword. They’re testing whether you understand the mechanism, the lab pattern, the inheritance, and how to eliminate distractors that sound clotty but don’t fit. Let’s break it down the way the exam writes it: one best answer, and every other option is there to trap a specific misconception.

Tag: Heme/Onc > Hemostasis & Coagulation

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The Vignette (Classic Board Style)

A 28-year-old woman presents with unilateral leg swelling and pain 1 week after a long-haul flight. She has no prior medical problems but reports that her father had a “blood clot” in his 30s. She is not pregnant and is not taking oral contraceptives. Ultrasound confirms a proximal DVT. Coagulation studies show normal PT and normal aPTT. Which of the following best explains her predisposition?

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The Correct Answer: Factor V Leiden

What it is

Factor V Leiden is a point mutation in the factor V gene that makes factor V resistant to cleavage (inactivation) by activated protein C (APC).

Why that causes thrombosis

Normally:

• Thrombin converts factor V → factor Va
• Factor Va is a cofactor for factor Xa in the prothrombinase complex, which generates more thrombin
• Activated protein C (APC) (with protein S) turns the system off by inactivating factor Va and factor VIIIa

With Factor V Leiden:

• APC can’t inactivate factor Va effectively
• Prothrombinase complex stays “on”
• More thrombin → more fibrin → hypercoagulability
• Clinical phenotype: venous thrombosis (DVT/PE), not platelet-type arterial clots

Step/USMLE high-yield facts

• Most common inherited thrombophilia in Caucasians
• Autosomal dominant with variable penetrance (heterozygotes are common; homozygotes have higher risk)
• Typical presentation: unprovoked or recurrent DVT/PE, often young, sometimes triggered (surgery, pregnancy, OCPs, immobility)
• Labs: PT and aPTT are typically normal
  (The problem is regulatory—inactivation—not baseline factor deficiency.)
• Testing clue: “APC resistance” assay abnormal; confirm with genetic testing

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How to Think Like the Test Writer: Why Each Distractor Is Wrong

Below are common answer choices that orbit the same concept (hypercoagulability) but differ in mechanism, labs, and clinical pattern.

Distractor 1: Protein C Deficiency

Tempting because: Protein C is in the same pathway as Factor V Leiden and causes venous thrombosis.

Key mechanism:

• Low protein C → less inactivation of Va and VIIIa → hypercoagulable

How to distinguish from Factor V Leiden:

• History may include:
  • Warfarin-induced skin necrosis (classic board pearl)
  • Family history of thrombosis
• Labs: PT/aPTT often normal at baseline (so you need clinical clues)
• Big board hook: Warfarin knocks out vitamin K–dependent factors (II, VII, IX, X) and proteins C/S, and protein C drops first due to short half-life → transient hypercoagulability early in therapy.

Exam tip: If the stem mentions starting warfarin without heparin bridging and then skin necrosis → think protein C deficiency, not Factor V Leiden.

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Distractor 2: Protein S Deficiency

Tempting because: Also a venous thrombophilia; also relates to APC.

Key mechanism:

• Protein S is the cofactor for APC
• Low protein S → APC can’t effectively inactivate Va/VIIIa

Distinguishers:

• Can be hereditary or acquired
• Acquired causes are frequently tested:
  • Pregnancy
  • Estrogen therapy
  • Liver disease
  • Nephrotic syndrome (loss of proteins)
• Labs: Often normal PT/aPTT at baseline

Exam tip: If the question leans into pregnancy/OCPs with thrombosis and asks for a decreased anticoagulant factor → protein S is a favorite.

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Distractor 3: Antithrombin (ATIII) Deficiency

Tempting because: Another classic inherited thrombophilia with venous clots.

Key mechanism:

• Antithrombin inhibits thrombin (IIa) and factor Xa (and others)
• Deficiency → too much thrombin activity → thrombosis

The giveaway detail: Heparin resistance

• Heparin works by potentiating antithrombin
• If antithrombin is deficient, heparin has less effect → “heparin resistance” (need higher doses; anti-Xa levels won’t respond as expected)

Exam tip: DVT + poor response to heparin → think ATIII deficiency.

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Distractor 4: Antiphospholipid Syndrome (APS)

Tempting because: It’s hypercoagulable and common in question banks—and it messes with coag labs.

Key mechanism:

• Autoantibodies against phospholipid-binding proteins (e.g., anti–β2 glycoprotein I, anticardiolipin, lupus anticoagulant)
• Causes thrombosis + pregnancy morbidity (recurrent miscarriages)

The lab paradox:

• Prolonged aPTT due to interference with phospholipid-dependent assays
• But in vivo thrombosis occurs

Distinguishers:

• History of:
  • Recurrent miscarriages
  • Livedo reticularis
  • Stroke/DVT in young person
• Confirmatory testing:
  • Lupus anticoagulant tests (e.g., dilute Russell viper venom time) that do not correct with mixing
  • Anticardiolipin/anti–β2 glycoprotein I antibodies

Exam tip: If the stem says “prolonged aPTT” plus clots → APS beats Factor V Leiden.

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Distractor 5: Prothrombin Gene Mutation (G20210A)

Tempting because: Common inherited thrombophilia with venous thrombosis.

Key mechanism:

• Mutation increases prothrombin (factor II) levels → increased thrombin generation

Distinguishers:

• Similar clinical phenotype to Factor V Leiden: venous clots, often young, normal PT/aPTT
• You usually need the stem to specify “increased prothrombin levels” or the gene mutation.

Exam tip: If answer choices include both Factor V Leiden and prothrombin mutation, the question typically gives one extra hint—either APC resistance (FVL) or elevated prothrombin (G20210A).

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Distractor 6: von Willebrand Disease

Tempting because: It’s very common and tied to coagulation, but it’s a bleeding disorder.

Key mechanism:

• Impaired platelet adhesion (vWF binds platelets to exposed collagen via GpIb)
• Also stabilizes factor VIII → mild factor VIII deficiency

Clinical/labs:

• Mucocutaneous bleeding (epistaxis, menorrhagia, easy bruising)
• Prolonged bleeding time (or abnormal platelet function testing)
• aPTT may be prolonged (due to ↓ factor VIII), PT normal

Exam tip: If the patient is clotting (DVT/PE), vWD should feel wrong immediately.

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Distractor 7: Hemophilia A or B

Tempting because: Students memorize the intrinsic pathway and see “aPTT,” but hemophilia causes bleeding, not clots.

Key mechanism:

• Hemophilia A: factor VIII deficiency
• Hemophilia B: factor IX deficiency

Clinical/labs:

• Hemarthroses, deep tissue bleeding, delayed bleeding after procedures
• Prolonged aPTT, normal PT and bleeding time

Exam tip: DVT + normal aPTT should steer you away from hemophilia.

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Distractor 8: Disseminated Intravascular Coagulation (DIC)

Tempting because: It involves clotting and bleeding—but it’s an acute consumptive coagulopathy.

Key mechanism:

• Widespread coagulation activation → consumption of platelets and clotting factors

Clinical/labs:

• Sick patient (sepsis, trauma, malignancy, obstetric catastrophe)
• Prolonged PT and prolonged aPTT
• Low fibrinogen
• Elevated D-dimer
• Thrombocytopenia
• Schistocytes may be present

Exam tip: DIC is systemic illness + abnormal labs everywhere. Factor V Leiden is often a relatively well patient with a clot and normal PT/aPTT.

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Rapid Comparison Table (High-Yield)

N = normal; ± = sometimes

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What USMLE Actually Wants You to Say Out Loud

When you see: young patient + venous thrombosis + family history + normal PT/aPTT, your internal script should be:

• “This is an inherited thrombophilia.”
• “Normal PT/aPTT means not a factor deficiency in the classic pathways.”
• “Most common inherited thrombophilia is Factor V Leiden.”
• “Mechanism: APC can’t inactivate factor V → increased thrombin generation.”

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Quick Memory Hooks (No Fluff, Just Useful)

• Factor V Leiden: “V won’t Leave (be cleaved) by APC.”
• APS: “Lupus anticoagulant prolongs aPTT but causes clots.”
• Antithrombin deficiency: “Heparin needs AT—no AT, no heparin effect.”
• Protein C deficiency: “Warfarin skin necrosis = protein C problem until proven otherwise.”