Differential Diagnosis

The patient's symptoms of vomiting, diarrhea, fatigue, decreased urination, and a new leg rash, along with laboratory findings of marked anemia, thrombocytopenia, and schistocytes on blood smear, suggest a complex clinical picture. Here is a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Hemolytic Uremic Syndrome (HUS): This condition is characterized by the triad of hemolytic anemia (suggested by the presence of schistocytes), thrombocytopenia, and acute kidney injury (indicated by decreased urination). It often follows a diarrheal illness caused by E. coli O157:H7, which could be acquired from contaminated food, making it a strong candidate given the patient's history of eating local street food before the onset of symptoms.

• Other Likely Diagnoses

  • Thrombotic Thrombocytopenic Purpura (TTP): Similar to HUS, TTP presents with thrombocytopenia and microangiopathic hemolytic anemia. However, it typically includes neurological symptoms and fever, which are not mentioned. The presence of a rash could be considered part of the purpura aspect, but the primary distinction would be the absence of significant renal failure as seen in HUS.
  • Severe Gastroenteritis with Secondary Complications: While the primary issue seems to be gastrointestinal, the severity of the symptoms and the development of systemic complications like anemia and thrombocytopenia suggest that the condition has progressed beyond simple gastroenteritis. However, the initial presentation could still be consistent with a severe infection.

• Do Not Miss Diagnoses

  • Meningococcemia: This condition, caused by Neisseria meningitidis, can present with a rash, fever, and rapidly progressing shock. Although the primary symptoms do not align perfectly with the patient's presentation, the appearance of a new rash and the potential for severe, life-threatening disease make it crucial not to miss this diagnosis.
  • Disseminated Intravascular Coagulation (DIC): This is a condition characterized by both widespread clotting and bleeding in the vascular system, which could explain the thrombocytopenia and anemia. It can be triggered by severe infection, among other causes, and is a critical diagnosis to consider due to its high mortality rate if not promptly treated.

• Rare Diagnoses

  • Atypical Hemolytic Uremic Syndrome (aHUS): This is a rare condition similar to HUS but not typically preceded by diarrhea. It involves complement system dysregulation and can present with similar laboratory findings to HUS but often has a more chronic course and can be triggered by various factors, including genetic predispositions.
  • Thrombotic Microangiopathy (TMA) associated with other conditions: TMAs can be associated with conditions like HIV, malignancies, or certain medications. These would be less likely given the patient's age and presentation but should be considered if other diagnoses are ruled out, especially if there's an atypical response to treatment or an unusual combination of symptoms.