What is the primary consequence of drawing blood from a patient's arm too quickly?

Drawing blood from a patient's arm too quickly can indeed lead to the collapse of the vein. When blood is drawn rapidly, it can create excessive negative pressure within the vein, which may cause the vein to collapse. This occurs because the veins, especially smaller ones, are not as robust as arteries and can easily be overwhelmed by the sudden change in pressure. A collapsed vein may result in difficulties in successfully obtaining a blood sample and could also lead to patient discomfort or complications.

The other options highlight potential consequences of drawing blood, but they do not directly relate to the immediate effect of drawing blood too quickly. For example, fainting can occur from various factors, including anxiety or pain, but it is not a direct result of the speed at which blood is drawn. Infection is a risk in any blood draw but is influenced more by the cleanliness of the procedure rather than the speed of the draw. Lastly, if the tube is not filled, it usually indicates an issue with the collection technique or equipment, not necessarily the speed of the draw alone.