A 28-year-old man with a lifelong history of easy bruising, spontaneous bleeding into joints and muscles, and prolonged bleeding after dental extraction is found to have a prolonged aPTT with normal PT and platelet count. What is the most common cause of this bleeding disorder?

Explanation

## Diagnosis: Factor VIII Deficiency (Hemophilia A) ### Clinical Presentation Analysis The patient presents with: - **Lifelong history** of easy bruising and spontaneous bleeding - **Hemarthroses and muscle hematomas** (deep tissue bleeding) - Prolonged bleeding after trauma (dental extraction) - **Prolonged aPTT** with normal PT and platelet count - Male gender ### Why Factor VIII Deficiency is the Most Common Cause **High-Yield:** Hemophilia A (Factor VIII deficiency) is the **most common inherited bleeding disorder**, accounting for ~80% of all hemophilias and ~1 in 5,000 male births. It is an X-linked recessive disorder, explaining the male predominance. ### Coagulation Pathway and aPTT Interpretation ```mermaid flowchart TD A[Intrinsic Pathway Activation]:::outcome --> B[Factor XII, XI, IX, VIII] B --> C[Tenase Complex: IXa + VIIIa + X + Ca2+]:::action C --> D[Factor X → Xa]:::action D --> E[Common Pathway]:::outcome E --> F[Prothrombin → Thrombin]:::action F --> G[Fibrinogen → Fibrin Clot]:::outcome H[aPTT measures] --> B I[PT measures] --> J[Extrinsic: Factor VII] K[Normal PT + Prolonged aPTT] --> L[Intrinsic pathway defect]:::decision L --> M[Factor VIII, IX, XI, or XII deficiency]:::outcome ``` **Key Point:** aPTT (activated partial thromboplastin time) measures the **intrinsic and common pathways**. A prolonged aPTT with normal PT indicates a defect in the intrinsic pathway (Factors VIII, IX, XI, XII) or common pathway (Factors II, V, X, fibrinogen). ### Differential Diagnosis: Intrinsic Pathway Defects | Factor | Inheritance | Frequency | Clinical Features | Bleeding Pattern | |--------|-------------|-----------|-------------------|------------------| | **VIII (Hemophilia A)** | **X-linked recessive** | **~80% of hemophilias** | Spontaneous bleeding, hemarthroses, muscle hematomas | **Deep tissue** | | V | Autosomal recessive | Rare (~1:1,000,000) | Mucocutaneous + deep tissue | Mixed | | II (Prothrombin) | Autosomal recessive | Rare (~1:500,000) | Mucocutaneous + deep tissue | Mixed | | XII | Autosomal recessive | Rare (~1:1,000,000) | **Asymptomatic despite ↑aPTT** | **None** | **Clinical Pearl:** The **combination of lifelong symptoms, male gender, and hemarthroses** is pathognomonic for X-linked hemophilia A. Factor VIII deficiency causes **spontaneous and post-traumatic bleeding into joints and muscles** because Factor VIII is critical for the intrinsic tenase complex. ### Severity Classification of Hemophilia A **Mnemonic: SEVERITY TIERS — "SEVERE, MODERATE, MILD"** - **Severe:** Factor VIII <1% (spontaneous bleeding, frequent hemarthroses) - **Moderate:** Factor VIII 1–5% (bleeding with minor trauma) - **Mild:** Factor VIII 5–40% (bleeding with significant trauma or surgery) This patient's **spontaneous bleeding and hemarthroses** suggest **severe hemophilia A** (Factor VIII <1%). ### Why aPTT is Prolonged but PT is Normal **Key Point:** - **aPTT** measures intrinsic (XII, XI, IX, VIII) + common pathway (II, V, X, fibrinogen) - **PT** measures extrinsic (VII) + common pathway (II, V, X, fibrinogen) - Factor VIII deficiency **only affects the intrinsic pathway**, so: - aPTT is **prolonged** (intrinsic pathway blocked) - PT is **normal** (extrinsic pathway intact) - Platelet count is **normal** (primary hemostasis intact) --- ## Why Each Distractor is Wrong **Factor V deficiency:** While Factor V deficiency causes prolonged aPTT, it is **extremely rare** (~1:1,000,000) compared to hemophilia A (~1:5,000). Additionally, Factor V deficiency is **autosomal recessive**, so it affects both males and females equally. The clinical presentation of a young male with lifelong hemarthroses is far more consistent with X-linked hemophilia A. **Factor II deficiency (Prothrombin deficiency):** Factor II deficiency is also **extremely rare** (~1:500,000) and causes **prolonged both PT and aPTT** (because Factor II is in the common pathway). This patient has **normal PT**, which excludes Factor II deficiency. Additionally, Factor II deficiency causes mixed mucocutaneous and deep tissue bleeding, not the predominantly deep tissue bleeding (hemarthroses) seen here. **Factor XII deficiency:** Factor XII deficiency is a **laboratory curiosity** — it causes **prolonged aPTT but NO clinical bleeding** because Factor XII is not essential for hemostasis in vivo (it is involved in contact activation, not physiologic coagulation). This patient has **lifelong spontaneous bleeding and hemarthroses**, which is incompatible with Factor XII deficiency. Factor XII deficiency is often discovered incidentally on routine coagulation testing.