Final answer:
To examine a patient's dorsal cavity, the medical assistant should position the patient prone, face-down, allowing access to the patient's back, which includes the cranial and spinal cavities.
Step-by-step explanation:
When a medical provider instructs a medical assistant to position a patient for an examination of the patient's dorsal cavity, the medical assistant should position the patient in a way that provides the best access to the dorsal cavity. The dorsal cavity includes the cranial and spinal cavities, so to examine this area, the patient is typically positioned prone, meaning lying face-down. This position allows the provider clear access to the back of the patient for examination and possible diagnostic procedures.