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The answer is c, Idiopathic (immune) thrombocytopenic purpura.

In children, idiopathic or immune thrombocytopenic purpura (ITP) is the most common form of thrombocytopenic purpura. In most cases, a preceding viral infection can be noted. No diagnostic test identifies this disease; exclusion of the other diseases listed in the question is necessary.

In this disease, the platelet count is frequently less than 20,000/µL, but other laboratory tests yield essentially normal results, including the bone marrow aspiration (if done). For ITP, platelets are sequestered and destroyed at the spleen by the reticuloendothelial system (RES) that binds self-immunoglobulins attached to the platelet.

Treatment for ITP consists of observation and/or gamma globulin and steroids. Splenectomy is reserved for the most severe and chronic forms. Exogenous IV gamma globulin can work to saturate the RES binding sites for platelet-bound self-immunoglobulin. Thus, there is less platelet uptake and destruction by the spleen.

Aplastic anemia is unlikely if the other cell lines are normal. Von Willebrand disease might be expected to present with bleeding and not just bruising. It is unlikely that acute leukemia would present with thrombocytopenia only. Thrombotic thrombocytopenic purpura is rare in children.


Edit:
bone marrow aspiration = Ponction-aspiration de la moelle osseuse
steroids = corticoides