A medullated nerve fiber is also called as myelinated nerve fiber. The nerve fibers, which are covered by the myelin sheath are called as medullated nerve fibers. Myelin sheath is derived from Schwann cells. Myelin sheath is made up of alternate concentric layers of lipids and proteins. Myelinated nerve fiber also shows the presence of neurolemma. It covers the nerve fiber.
So, the correct answer is 'Both A and B'