Answer: The next most appropriate step would be a LUMBAR PUNCTURE.
Explanation:
A LUMBAR PUNCTURE is a medical procedure that is usually carried out by a trained health personnel. It is a procedure that is carried out in the lumbar region of the spinal cord to access and obtain the cerebrospinal fluid which is used for further diagnosis of an underlying disease or illness.
A LUMBAR PUNCTURE is often adviced when the symptoms of a disease is related to the central nervous system. From the symptoms observed from the 19-year-old woman at the emergency department which includes:
--> generalized headache which is increasing in intensity,
--> blurred vision,
--> intermittent diplopia,
--> vague dizziness.
--> bilateral papilledema and
--> normal MRI of the brain.
Since the MRI of the brain is normal, the most appropriate step to be taken by the health care provider is a lumbar puncture which enables a direct access to the cerebrospinal fluid. This would help to rule out an elevated intracranial pressure which may be the cause of the generalized headache which is increasing in intensity.