Final answer:
The most likely reason for the arrested blood flow in an EDTA tube during a venipuncture after other tubes have been properly filled is the collapse of the patient's vein.
Step-by-step explanation:
When a phlebotomy technician experiences an interruption in blood flow during the collection of an EDTA tube, after successfully filling other tubes like a citrate tube and a serum separator tube (SST), the most likely explanation is that the patient's vein has collapsed (Option A).
Venipuncture complications could lead to a vein collapsing, especially if the patient's veins are fragile or if the vacuum within the tube has already been compromised. However, given that prior tubes were filled correctly, a collapse seems more probable than a compromised vacuum. A clot forming in the needle (Option B) might also be possible but is less likely if proper technique was used and due to the presence of anticoagulants in the tubes. The vein being empty (Option D) is highly unlikely as veins don't 'run out' of blood as long as the circulatory system is functioning. Therefore, the sum of evidence points towards a vein collapse as the culprit for the halted blood flow.