A 28-year-old woman presents with a 3-day history of severe epistaxis and gum bleeding. She reports easy bruising over the past week. On examination, she has petechiae on her lower extremities and oral mucosa. Laboratory investigations reveal: hemoglobin 9.2 g/dL, platelet count 8,000/μL, PT 12 sec (normal), aPTT 32 sec (normal), fibrinogen 280 mg/dL (normal). Bone marrow biopsy shows normocellular marrow with adequate megakaryocytes. What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis This patient presents with severe thrombocytopenia (8,000/μL) with mucocutaneous bleeding and petechiae, but **normal coagulation studies** (PT, aPTT, fibrinogen) and a **normocellular bone marrow with adequate megakaryocytes**. ## Diagnostic Criteria for ITP **Key Point:** Immune thrombocytopenia is characterized by: 1. Isolated thrombocytopenia (platelet count <150,000/μL) 2. Normal or increased megakaryocytes on bone marrow examination 3. Normal coagulation parameters 4. Absence of splenomegaly or lymphadenopathy (unless secondary) 5. No evidence of other causes of thrombocytopenia ## Differential Diagnosis Table | Feature | ITP | TTP | DIC | HUS | |---------|-----|-----|-----|-----| | **Platelet count** | Very low | Very low | Very low | Very low | | **PT/aPTT** | Normal | Normal | Prolonged | Normal | | **Fibrinogen** | Normal | Normal | Low | Normal | | **Schistocytes** | Absent | Present (MAHA) | Present (MAHA) | Present (MAHA) | | **Renal dysfunction** | No | Yes (AKI) | Yes | Yes (prominent) | | **Fever** | No | Yes (pentad) | No | No | | **Megakaryocytes** | Adequate/↑ | Normal/↓ | Normal/↓ | Normal/↓ | | **Hemolysis** | No | Yes | Yes | Yes | ## Why This Is ITP **High-Yield:** The **normal coagulation cascade** (PT, aPTT, fibrinogen) rules out consumptive coagulopathies (TTP, DIC). The **adequate megakaryocytes** on bone marrow indicate peripheral destruction, not bone marrow failure. The **absence of hemolytic anemia features** (no schistocytes mentioned, normal hemoglobin relatively preserved) excludes microangiopathic hemolytic anemias (TTP, HUS). **Clinical Pearl:** ITP is a diagnosis of exclusion. The presence of adequate megakaryocytes with severe thrombocytopenia is pathognomonic for immune-mediated platelet destruction. ## Pathophysiology In ITP, autoantibodies (primarily IgG) bind to platelet surface antigens (GPIIb/IIIa, GPIb/IX), leading to: - Complement-mediated lysis - Fc-receptor-mediated phagocytosis by splenic macrophages - Direct platelet destruction [cite:Robbins 10e Ch 14]