Assume that a DGS-like phenotype was produced in a specific “gene-knockout” mouse, one lacking expression of HA3, a Hox gene known to be involved in developmental regulation in the mouse. The phenotype of the HA3 knockout can be ascertained by A) a bone marrow biopsy. B) an assay for environmental agents known to cause birth defects. C) a chest X-ray. D) the measurement of the proportion of CD4 cells to total lymphocytes. E) an autopsy examination of the adrenal glands.