What should be done immediately if a patient presents with symptoms of temporal arteritis?

The correct choice is to start high-dose corticosteroids because temporal arteritis, also known as giant cell arteritis, is a condition that requires immediate treatment to prevent serious complications, such as vision loss. This condition involves inflammation of the blood vessels, particularly the temporal arteries, and symptoms often include headaches, scalp tenderness, and visual disturbances.

High-dose corticosteroids are the first-line treatment and need to be initiated as soon as the diagnosis is suspected, even before confirmatory tests such as a biopsy or imaging studies are completed. The use of corticosteroids helps to rapidly control inflammation and reduce the risk of ischemic complications.

In the context of the other options, scheduling a follow-up MRI or referring to a rheumatologist would not address the immediate need for intervention. While monitoring symptoms is important, it does not provide the necessary urgent treatment needed for this potentially sight-threatening condition. Therefore, initiating high-dose corticosteroids is the most appropriate and timely action.