You’re cruising through a heme Q-bank and suddenly the coag cascade shows up—again. It feels like “just memorize the pathways,” but the real test is recognizing where the pathway breaks based on PT/PTT patterns, bleeding history, and mixing studies. Let’s do this like a classic USMLE vignette: pick the right answer, then make every distractor teach you something.
Tag: Heme/Onc > Hemostasis & Coagulation
The Vignette (Q-bank style)
A 22-year-old man presents with recurrent bleeding after dental procedures and frequent nosebleeds since childhood. He has no joint swelling. His mother reports similar bleeding problems. Vital signs are normal. Exam shows scattered ecchymoses. Labs:
| Test | Result |
|---|---|
| Platelet count | Normal |
| Bleeding time / PFA-100 | Increased |
| PT | Normal |
| aPTT | Increased |
| Mixing study (1:1 with normal plasma) | Corrects |
Which of the following is the most likely underlying abnormality?
A. Decreased factor VIII activity due to autoantibody inhibitor
B. Defective platelet GPIIb/IIIa receptor
C. Defective platelet adhesion due to impaired vWF function
D. Vitamin K deficiency
E. Factor XI deficiency (Hemophilia C)
Step-by-Step: What the stem is really telling you
1) Mucocutaneous bleeding points to primary hemostasis
- Epistaxis, easy bruising, gingival bleeding, menorrhagia = platelets/vWF classically
- Deep tissue bleeding (hemarthroses, muscle hematomas) = coag factors (secondary hemostasis)
This patient has mucocutaneous bleeding and no hemarthroses, pushing you toward platelet-type problems.
2) Platelet count normal + increased bleeding time/PFA
- Not thrombocytopenia
- Something wrong with platelet function or vWF-mediated adhesion
3) aPTT increased + PT normal
That’s the intrinsic pathway: factors XII, XI, IX, VIII.
So why would a platelet-type disorder also prolong aPTT?
Because vWF stabilizes factor VIII. Low/defective vWF → low factor VIII → ↑ aPTT.
4) Mixing study corrects
- Corrects = factor deficiency (you supplied the missing factor)
- Does NOT correct = inhibitor (autoantibody, lupus anticoagulant)
Correct Answer: C. Defective platelet adhesion due to impaired vWF function (von Willebrand disease)
Why vWD fits perfectly
von Willebrand disease causes:
- Defective platelet adhesion (vWF bridges platelet GPIb to exposed subendothelial collagen)
- Decreased factor VIII stability → may increase aPTT
- Mucocutaneous bleeding (epistaxis, gingival bleeding, menorrhagia, post-dental bleeding)
- Often autosomal dominant, so family history can show up in multiple generations
High-yield labs in vWD
- Platelet count: usually normal
- PFA/bleeding time: increased
- PT: normal
- aPTT: normal or increased
- Mixing study: corrects (if prolonged due to low factor VIII)
High-yield treatment
- Desmopressin (DDAVP): increases release of vWF and factor VIII from Weibel-Palade bodies (endothelium)
- For severe cases: vWF-containing factor concentrates
Now: Why Every Distractor Is Wrong (and what it teaches)
A. Decreased factor VIII activity due to autoantibody inhibitor (Acquired hemophilia A)
This is a mixing study trap.
What you’d expect instead:
- Often older adults or postpartum; associated with autoimmune disease or malignancy
- Bleeding can be severe (soft tissue, mucosal, GI), but pattern is secondary hemostasis
- Isolated ↑ aPTT
- Mixing study does NOT correct (inhibitor neutralizes factor VIII in both patient + normal plasma)
Key distinction:
- vWD: aPTT may be ↑ but mixing corrects
- Acquired hemophilia A: aPTT ↑ and mixing fails to correct
B. Defective platelet GPIIb/IIIa receptor (Glanzmann thrombasthenia)
Glanzmann = aggregation problem, not adhesion.
What you’d expect instead:
- Mucocutaneous bleeding, yes
- Platelet count: normal
- Bleeding time/PFA: increased
- PT and aPTT: normal (coag factors are fine)
- Abnormal platelet aggregation studies: no aggregation with ADP, epinephrine, collagen (but normal ristocetin test)
High-yield one-liner:
- GPIIb/IIIa binds fibrinogen → platelet-platelet bridging (aggregation)
So why it’s wrong here: the vignette has ↑ aPTT, which Glanzmann doesn’t cause.
D. Vitamin K deficiency
Vitamin K affects the “1972” factors:
- II, VII, IX, X + proteins C and S
What you’d expect:
- PT increases first (factor VII has the shortest half-life)
- Later: both PT and aPTT can be elevated
- Bleeding pattern is more secondary hemostasis
- Risk factors: newborns (no gut flora), malabsorption, prolonged broad-spectrum antibiotics, warfarin use
So why it’s wrong here:
- The patient has normal PT and a strong primary-hemostasis story.
Extra Step point: Warfarin initially causes hypercoagulability due to early drop in protein C → skin necrosis risk.
E. Factor XI deficiency (Hemophilia C)
Factor XI deficiency can present more subtly than hemophilia A/B and often causes bleeding after surgery.
What you’d expect:
- ↑ aPTT, PT normal
- Mixing study: corrects
- Bleeding after procedures can occur
- Classically more common in Ashkenazi Jewish patients
- Not typically hemarthroses like hemophilia A/B (variable phenotype)
So why it’s still not best:
- The vignette emphasizes mucocutaneous bleeding + increased bleeding time/PFA, which is primary hemostasis and points strongly to vWD over factor XI deficiency.
Rapid-Review: Hemostasis in One Table
| Feature | Primary Hemostasis | Secondary Hemostasis |
|---|---|---|
| Main players | Platelets, vWF, endothelium | Coagulation factors |
| Typical bleeding | Mucocutaneous (petechiae, epistaxis, menorrhagia) | Deep (hemarthroses, muscle hematomas) |
| Key tests | Platelet count, PFA/bleeding time | PT, aPTT |
| Example disorders | vWD, Glanzmann, Bernard-Soulier | Hemophilia A/B, vitamin K deficiency |
Intrinsic vs Extrinsic: What PT/aPTT Actually Map To
Intrinsic pathway (aPTT)
Factors: XII, XI, IX, VIII
Also affected by: heparin, inhibitors (e.g., acquired hemophilia), lupus anticoagulant
Extrinsic pathway (PT)
Factor: VII
Affected by: warfarin, early vitamin K deficiency, liver disease
Common pathway (both)
Factors: X, V, II (prothrombin), I (fibrinogen)
Mixing Studies: The USMLE Shortcut Algorithm
When PT or aPTT is prolonged:
- Mixing corrects → factor deficiency
- Hemophilias, vitamin K deficiency (eventually), liver disease (often), DIC (complex)
- Mixing does not correct → inhibitor
- Lupus anticoagulant (thrombosis > bleeding)
- Factor VIII inhibitor (acquired hemophilia A)
Take-Home: Why the Right Answer Is vWD
This vignette is engineered to reward you for combining:
- Mucocutaneous bleeding (primary hemostasis)
- Normal platelet count but increased PFA (qualitative platelet/vWF issue)
- Possible ↑ aPTT (factor VIII destabilized)
- Correction with mixing (deficiency, not inhibitor)
- Family history (often AD inheritance)
When you see post-dental bleeding + epistaxis + ↑ PFA + maybe ↑ aPTT, think von Willebrand disease first.