Final answer:
The presence of an Auer rod is the definitive sign that confirms a blast of unknown lineage is myeloid. Auer rods are specific to myeloblasts, which are part of the myeloid lineage. While other features may suggest myeloid origin, they are not exclusive to it.
Step-by-step explanation:
To determine whether a blast of unknown lineage is myeloid, several indicators can be observed. Among the options provided, the presence of an Auer rod confirms a myeloid lineage. Auer rods are distinctive needle-like inclusions primarily seen in myeloblasts which are indicative of acute myeloid leukemia (AML). Other features such as basophilia, toxic granulation, and leukocytosis can be associated with myeloid cells; however, they are not exclusive to them and could be a result of other conditions as well. Considering granulocytes and their lineage, basophils arise from myeloid stem cells and are a type of granular leukocyte. But to directly answer the question, Auer rod presence is the definitive sign of a myeloid lineage blast cell.