Final answer:
For a CBC test, a medical assistant should select a lavender tube, which contains EDTA anticoagulant necessary for preventing blood clotting and preserving cells for accurate analysis.
Step-by-step explanation:
When a medical assistant is obtaining samples for a Complete Blood Count (CBC) test, the appropriate tube to select is the lavender tube. Lavender tubes contain an anticoagulant, usually ethylenediaminetetraacetic acid (EDTA), which prevents clotting and is essential for a CBC test that includes a count of different types of blood cells such as white blood cells, red blood cells, and platelets, as well as measurements of hemoglobin and hematocrit. The anticoagulant in the lavender tube ensures that blood cells are preserved in their natural state for an accurate analysis.