Final answer:
Perform a standing echocardiogram to evaluate mitral valve prolapse.
Step-by-step explanation:
When the patient has clear evidence for mitral valve prolapse but the resting echo is normal, the next step is to perform a standing echocardiogram (option c). This is because the standing position can sometimes unmask changes in the mitral valve that are not present in the supine position. It can help to assess the severity of the mitral valve prolapse and guide further management. Raising the patient's legs (option a) or performing a cold pressor test (option b) are not indicated in this situation and will not provide the necessary information to evaluate the mitral valve prolapse.