What is the likely diagnosis for a 10-year-old girl with red urine, anemia, and a positive Donath-Landsteiner test after a recent upper respiratory infection?

The diagnosis of paroxysmal cold hemoglobinuria (PCH) is supported by the symptoms and test results provided in the case of the 10-year-old girl. PCH typically presents with hemolytic anemia, which can indeed manifest as red urine due to the presence of hemoglobin, especially after episodes of exposure to cold temperatures.

The positive Donath-Landsteiner test is also a critical piece of evidence. This test is specifically associated with PCH, as it reveals the presence of anti-P (Donath-Landsteiner antibodies) that activate complement in cold temperatures. In this condition, hemolysis occurs when warm blood returns to the body, leading to the destruction of red blood cells.

The context of a recent upper respiratory infection further strengthens this diagnosis, as PCH is commonly associated with viral infections, especially in pediatric patients. In summary, all of these factors—the presence of hemoglobinuria, anemia, the positive Donath-Landsteiner test, and the recent viral illness—coalesce to make paroxysmal cold hemoglobinuria the most likely diagnosis.