American Medical Technologist (AMT) Practice Exam 2025 – All-in-One Resource to Guarantee Exam Success!

The scenario in which blood begins to flow and then stops upon pushing the evacuated tube into the needle suggests an issue with how the needle is positioned in relation to the vein. When the bevel of the needle rests against the wall of the vein, it may occlude the lumen, preventing blood flow. This occurs because the beveled edge can create a barrier that obstructs the blood from entering the tube.

In contrast, the other factors discussed do not directly correlate with the immediate cessation of blood flow after positioning the tube. Inadequate vacuum in the tube would more likely affect the initial drawing of blood rather than cause flow to stop suddenly once initiated. Poor patient positioning can affect blood collection but is less likely to cause the described immediate effect if blood is already flowing. Excessive tourniquet pressure can impede blood flow, but again, if blood has already started to flow, removing the obstruction of the tourniquet would typically restore flow. Thus, the position of the needle itself, specifically the bevel resting against the vein wall, is the most likely reason for the described occurrence.