Final answer:
To achieve the most complete fluid distribution to the upper extremities during embalming of an autopsied decedent, the axillary artery should be used as it is a direct continuation of the subclavian artery and provides proximal and distal fluid distribution.
Step-by-step explanation:
When embalming an autopsied decedent, the practitioner should use the axillary artery to obtain the most complete fluid distribution to the upper extremities. This artery continues from where the subclavian artery exits the thorax and enters the axillary region, being a direct continuation into the arm. As such, it will ensure the delivery of embalming fluid both proximally and distally, reaching the arm's various regions including the brachial, radial, and ulnar arteries which further supply the wrist and hand areas.
The brachial artery is the continuation of the axillary artery into the upper arm and while it does supply a large portion of the arm, it is more distal than the axillary artery. For a thorough distribution, starting with the axillary artery is preferable.