Differential Diagnosis for a 5-year-old girl with low-grade fever, bloody diarrhea, lethargy, and scattered petechiae

• Single most likely diagnosis:
  • Hemolytic Uremic Syndrome (HUS) due to Shiga toxin-producing E. coli (STEC) infection. The presence of Shiga toxin in the stool culture, along with symptoms of bloody diarrhea, lethargy, and scattered petechiae, points towards HUS, a condition characterized by the triad of hemolytic anemia, acute kidney injury, and thrombocytopenia. On a peripheral smear, schistocytes (fragmented red blood cells) would be expected due to the mechanical damage caused by the microangiopathic hemolytic anemia associated with HUS.
• Other Likely diagnoses:
  • Thrombotic Thrombocytopenic Purpura (TTP): Although less common in children and not directly linked to Shiga toxin, TTP could present with similar symptoms, including thrombocytopenia and microangiopathic hemolytic anemia, thus also showing schistocytes on the peripheral smear.
  • Severe bacterial infection: Other bacterial infections could cause similar symptoms, including fever, diarrhea, and lethargy, but the presence of Shiga toxin specifically points towards STEC.
• Do Not Miss diagnoses:
  • Meningococcemia: This condition, caused by Neisseria meningitidis, can present with fever, lethargy, and petechiae or purpura. It's a medical emergency requiring prompt treatment. While the stool culture result does not directly suggest meningococcemia, the presence of petechiae and severe illness warrants consideration. Basophilic stippling might be seen in cases of lead poisoning or other conditions but is not typically associated with acute infections like meningococcemia.
  • Disseminated Intravascular Coagulation (DIC): This is a condition characterized by both widespread clotting and bleeding in the vascular system, which could be triggered by severe infection. It might show schistocytes among other abnormalities on the peripheral smear.
• Rare diagnoses:
  • Other infections that could cause similar symptoms, such as certain viral infections or less common bacterial pathogens, are less likely given the specific finding of Shiga toxin.
  • Negri bodies are associated with rabies and would not be relevant to this clinical presentation.
  • Target cells are seen in conditions like sickle cell disease or other hemoglobinopathies, which are not suggested by the acute presentation of this case.