🧠 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 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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