The potential energy source for an athlete sprinting the 40-yard dash is B) phosphocreatine.
Phosphocreatine (PCr) is a high-energy molecule that can be rapidly broken down to produce adenosine triphosphate (ATP), which is the primary energy source for muscle contractions. During short bursts of high-intensity activity, such as sprinting, the demand for ATP is very high, and the body relies on PCr as a quick source of energy to replenish ATP levels in the muscle cells.
Myoglobin is an oxygen-binding protein that helps transport oxygen to the muscle fibers, but it is not a source of energy for muscle contractions. Creatine kinase is an enzyme that catalyzes the conversion of PCr to ATP, but it is not a direct source of energy. Ryanodine receptors are calcium ion channels in the muscle cells that play a role in muscle contraction, but they are not a source of energy.
Therefore, the correct answer is B) phosphocreatine.