The most likely diagnosis is hypereosinophilic syndrome (HES). This patient presents with cardiac complications of HES, an elevated eosinophil count (>1500/µL [1.5 × 10⁹/L]) without a secondary cause, and evidence of organ involvement. The causes of eosinophilia are described in the CHINA mnemonic (connective tissue diseases, helminthic infection, idiopathic [HES], neoplasia, allergy). The most common organs affected by HES include the skin, lungs, gastrointestinal tract, and heart. HES may or may not be associated with mutations in the tyrosine kinase receptor gene, but such involvement is important to identify, because treatment with imatinib is effective in such cases.

Acute eosinophilic leukemia is characterized by increases in immature eosinophils in blood and bone marrow and infiltration of tissues with immature eosinophils. Like HES, cardiac dysfunction can develop, but findings of an acute leukemic syndrome are pronounced and include anemia, thrombocytopenia, and infection.

Chronic myeloid leukemia (CML) is a clonal hematopoietic stem cell disorder characterized by myeloid proliferation. In chronic-phase CML, the leukocyte count is high, the hemoglobin level is low or normal, and the platelet count is normal or high. CML may be associated with an absolute eosinophilia, but tissue or organ dysfunction is not common. Detection of the (9;22) translocation by routine cytogenetics or fluorescence in situ hybridization or of the BCR-ABL fusion transcript by reverse transcriptase-polymerase chain reaction is diagnostic.

Systemic mastocytosis with eosinophilia is characterized by urticaria pigmentosa, a unique identifying clinical finding. Urticaria pigmentosa findings include pruritic yellow to red or brown macules, papules, plaques, and nodules. The most common noncutaneous findings are gastrointestinal and include symptoms such as abdominal pain, diarrhea, nausea, and vomiting.