A 52-year-old man with newly diagnosed acute myeloid leukemia (AML) is started on chemotherapy. A bone marrow aspirate is performed 48 hours after the first dose. Morphologically, numerous leukemic blasts show cell shrinkage, chromatin condensation, and fragmentation into membrane-bound apoptotic bodies. Which investigation would be most specific to confirm intrinsic (mitochondrial) pathway apoptosis in these cells?