What is the most likely diagnosis for a previously healthy 4-year-old girl presenting with pallor, truncal bruising, and diffuse lymphadenopathy?

The presentation of a previously healthy 4-year-old girl with pallor, truncal bruising, and diffuse lymphadenopathy strongly points to acute lymphoblastic leukemia (ALL) as the most likely diagnosis.

In children, ALL is characterized by the overproduction of immature white blood cells, which can lead to symptoms like anemia (indicated by pallor), bleeding or bruising due to thrombocytopenia (low platelet count), and lymphadenopathy resulting from the infiltration of leukemic cells into the lymph nodes. The combination of these symptoms, particularly the bruising and lymphadenopathy, suggests a hematologic process rather than a more localized or benign condition.

While IgA vasculitis, Henoch-Schönlein purpura, and iron deficiency anemia could present with some overlapping symptoms, they do not typically encompass the full range of signs described. For example, IgA vasculitis often presents with palpable purpura and is usually associated with abdominal pain rather than diffuse lymphadenopathy. Henoch-Schönlein purpura also features similar rashes and symptoms but usually shows more classic signs of kidney involvement. Iron deficiency anemia generally presents with pallor but does not typically cause bruising or lymphadenopathy to the extent observed