Final answer:
For an examination of the patient's dorsal cavity, the medical assistant should place the patient in the prone position, which is a face-down orientation suitable for dorsal assessments.
Step-by-step explanation:
If a medical provider instructs a medical assistant to position a patient for an examination of the patient's dorsal cavity, the assistant should place the patient in the prone position. The prone position describes a face-down orientation, which is necessary when assessing the dorsal cavity that houses the brain and spinal cord. The supine position, where the patient lies face-up, is not appropriate for examinations that focus on the dorsal aspects of the body.