Which action best allows for vein accessibility during venipuncture?

Applying a tourniquet appropriately is crucial for enhancing vein accessibility during venipuncture. When a tourniquet is applied above the intended puncture site, it restricts venous blood flow while allowing arterial blood to continue to flow toward the arm. This results in the engorgement of the veins, making them more prominent and easier to locate and puncture. The engorgement of veins can reduce the difficulty of finding a suitable site for venipuncture, which is particularly beneficial when dealing with patients who may have small or difficult-to-palpate veins.

The other options contribute to patient comfort or procedural efficiency but do not directly enhance vein accessibility as effectively. For instance, having the patient lie flat might help in some cases by reducing anxiety or discomfort, but it does not specifically improve the visibility and palpability of veins. Choosing a smaller needle and using a larger collection tube are considerations that may affect the ease of drawing blood or the volume collected but do not influence the accessibility of the veins themselves.