Which finding best distinguishes Hemophilia A from von Willebrand disease in a patient presenting with mucosal bleeding and a prolonged aPTT?

Question

Accepted Answer

C. Abnormal ristocetin-induced platelet aggregation (RIPA). ## Distinguishing Hemophilia A from von Willebrand Disease

### Key Discriminator: Platelet Function Testing

**Key Point:** Abnormal ristocetin-induced platelet aggregation (RIPA) is the hallmark finding that distinguishes von Willebrand disease from Hemophilia A. This test directly assesses von Willebrand factor's role in platelet adhesion.

### Comparative Table

| Feature | Hemophilia A | von Willebrand Disease |
|---------|--------------|------------------------|
| **aPTT** | Prolonged | Prolonged (variable) |
| **Bleeding time** | Normal | Abnormal/prolonged |
| **Platelet count** | Normal | Normal |
| **RIPA** | Normal | Abnormal (↓ or absent) |
| **Factor VIII** | Markedly ↓ | Normal or mildly ↓ |
| **vWF antigen** | Normal | Reduced |
| **vWF activity** | Normal | Reduced |
| **Inheritance** | X-linked recessive | Autosomal dominant (Type 1) |

### Why RIPA Is the Discriminator

1. **Mechanism:** Ristocetin is an antibiotic that causes platelet agglutination only in the presence of functional von Willebrand factor. In vWD, deficient or dysfunctional vWF prevents this platelet response.

2. **In Hemophilia A:** Platelet function is completely normal because the defect is in factor VIII (a coagulation factor), not in platelet adhesion or vWF.

3. **Clinical Significance:** While both present with prolonged aPTT and mucosal bleeding, the platelet function defect in vWD creates an additional bleeding diathesis affecting primary hemostasis.

**High-Yield:** RIPA is the gold-standard confirmatory test for von Willebrand disease and is always normal in Hemophilia A.

**Clinical Pearl:** Patients with vWD often have abnormal bleeding time (reflecting platelet dysfunction), whereas Hemophilia A patients have normal bleeding time despite severe bleeding tendency — a seemingly paradoxical finding that hinges on the distinction between primary and secondary hemostasis.

Answer

A. Autosomal recessive inheritance pattern

Answer

B. Isolated factor VIII deficiency with normal von Willebrand factor antigen

Answer

D. Normal bleeding time and normal platelet count

Which finding best distinguishes Hemophilia A from von Willebrand disease in a patient presenting with mucosal bleeding and a prolonged aPTT?

Explanation

Distinguishing Hemophilia A from von Willebrand Disease

Key Discriminator: Platelet Function Testing

Comparative Table

Why RIPA Is the Discriminator

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Feature	Hemophilia A	von Willebrand Disease
aPTT	Prolonged	Prolonged (variable)
Bleeding time	Normal	Abnormal/prolonged
Platelet count	Normal	Normal
RIPA	Normal	Abnormal (↓ or absent)
Factor VIII	Markedly ↓	Normal or mildly ↓
vWF antigen	Normal	Reduced
vWF activity	Normal	Reduced
Inheritance	X-linked recessive	Autosomal dominant (Type 1)