Which of the following is NOT a determining factor when determining the yield grade of a carcass?

The assessment of yield grade in a carcass is primarily focused on the amount of usable meat versus non-meat components such as fat and bone. While external fat, carcass weight, muscle thickness, and carcass conformation are generally significant in evaluating yield grade, one of these options is not considered a direct determining factor.

Yield grade evaluation largely includes internal traits that correlate with meat quantity, which typically focuses on factors such as the amount of fat and muscle, as well as the weight of the carcass. External fat does play a role in determining quality grade by influencing factors like marbling but is less central in the calculation of yield grade itself, which aims specifically at quantifying the lean meat yield.

Muscle thickness, carcass weight, and conformation are critical because they provide a clearer view of the meat's potential yield. In contrast, external fat primarily affects quality perceptions and is not a foundational aspect of yield grading, which is more concerned with the meat's overall volume and its ratios to fat and bone. This is why it stands out as the correct answer in this context.