Hemolytic Uremic Syndrome (HUS)
The most likely diagnosis is B. Hemolytic uremic syndrome (HUS), given the combination of thrombocytopenia, normal coagulation parameters (PT, PTT, INR), and preceding bloody diarrhea in a child.
Clinical Reasoning
Key Diagnostic Features Present
- Bloody diarrhea preceding thrombocytopenia: This classic prodrome strongly suggests HUS, particularly the typical Shiga toxin-associated form that follows gastrointestinal infection 1
- Isolated thrombocytopenia with normal PT/PTT/INR: This pattern is characteristic of platelet consumption disorders rather than coagulopathies 2
- Absence of rash: This helps exclude other thrombocytopenic conditions with cutaneous manifestations 3
Why Not the Other Options
ITP (Option A) is unlikely because:
- ITP typically presents with isolated thrombocytopenia WITHOUT preceding bloody diarrhea 4, 5
- ITP diagnosis requires exclusion of other causes, and the bloody diarrhea history points to a specific alternative etiology 4
- While ITP can occur in children, the gastrointestinal prodrome is not characteristic 5
TTP (Option C) is less likely because:
- TTP is extremely rare in children compared to HUS 2
- The clinical presentation with bloody diarrhea is more consistent with HUS than TTP 1
DIC (Option D) is excluded because:
- DIC characteristically shows prolonged PT and PTT due to consumption of clotting factors 2
- This patient has normal PT, PTT, and INR, which effectively rules out DIC 2
- Studies demonstrate that normal PT with profound thrombocytopenia has 92% specificity for TTP-HUS over DIC 2
Pathophysiology Supporting HUS Diagnosis
HUS involves severe renal endothelial cell injury that produces both platelet and red cell destruction 1. The thrombocytopenia results from platelet destruction with only minimal changes in fibrinogen turnover and fibrinolytic degradation products, explaining the normal coagulation parameters 1.
Clinical Pitfall to Avoid
Do not confuse isolated thrombocytopenia with normal coagulation studies with DIC. The presence of normal PT/PTT/INR essentially excludes DIC, as DIC involves consumption of clotting factors leading to prolonged coagulation times 2. The combination of thrombocytopenia with normal PT has high specificity for microangiopathic processes like HUS rather than consumptive coagulopathy 2.