🧠 Coagulation Cascade for USMLE Step 1: Understand the Intrinsic, Extrinsic, and Common Pathways

The coagulation cascade is a critical concept in USMLE Step 1, tying together biochemistry, physiology, and pathology.

Step 1 questions may present:

  • Bleeding disorders

  • Lab abnormalities (↑PT, ↑PTT)

  • Genetic mutations (e.g., Factor V Leiden)

To master it, understand the pathways, factors, and associated labs.

🧠 Coagulation Cascade for USMLE Step 1: Understand the Intrinsic, Extrinsic, and Common Pathways

🧪 Coagulation Pathways Overview

Pathway Trigger Key Factors Lab Test
Intrinsic Surface contact XII, XI, IX, VIII PTT
Extrinsic Tissue damage VII PT / INR
Common Pathway convergence X, V, II, I, XIII Both PT & PTT

💡 Mnemonic:TENET” for Twelve, Eleven, Nine, Eight, Ten (intrinsic → common)

🩸 Clotting Factors by Roman Numerals

Number Name Notes
I Fibrinogen Converted to fibrin (final clot)
II Prothrombin Converted to thrombin (IIa)
VII Stable factor Extrinsic; monitored via PT
VIII Anti-hemophilic A Hemophilia A (X-linked)
IX Anti-hemophilic B Hemophilia B (Christmas disease)
X Stuart-Prower Activates thrombin pathway

🧬 Intrinsic vs Extrinsic Pathways – Step 1 Tips

Feature Intrinsic Extrinsic
Initiation Collagen / endothelial injury Tissue factor (TF)
Lab Test PTT PT / INR
Drug Target Heparin Warfarin

🧪 Clinical Applications

Disorder Pathway Clue
Hemophilia A Intrinsic ↑PTT, normal PT
Vitamin K deficiency Extrinsic + Common ↑PT, ↑INR
DIC All pathways ↑PT, ↑PTT, ↓platelets, schistocytes
Liver failure All ↑PT and ↑PTT (no clotting factors)

🎯 Step 1 Mnemonics

  • “Play Tennis Inside” → PTT = Intrinsic

  • “Play Table Outside” → PT = Extrinsic

  • “Vitamin K = 1972” → Affects factors II, VII, IX, X

🧠 Sample USMLE Question

A 10-year-old boy has recurrent joint bleeds and prolonged PTT but normal PT. Which factor is deficient?
Answer: Factor VIII → Hemophilia A

💡 Final Summary Table

Pathway Lab Key Factors Drug
Intrinsic PTT XII, XI, IX, VIII Heparin
Extrinsic PT / INR VII Warfarin
Common Both X, V, II, I

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