This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.N.S, Subject name: Medicine
classic tetrad of Refsum's disease includes all of the following except? 
Select and explain. 
A. cerebellar ataxia, B. peripheral neuropathy, C. retinitis pigmentosa, D. normal or reduced CSF protein concentration
D: normal or reduced CSF protein concentration
Answer: D: normal or reduced CSF protein concentration 
Explanation: Refsum's disease can manifest in infancy to early adulthood with the classic tetrad of (1) peripheral neuropathy, (2) retinitis pigmentosa, (3) cerebellar ataxia, and (4) elevated CSF protein concentration. Most affected individuals develop progressive distal sensory loss and weakness in the legs leading to foot drop by their 20 s . Subsequently, the proximal leg and arm muscles may become weak. Patients may also develop sensorineural hearing loss, cardiac conduction abnormalities, ichthyosis, and anosmia. Serum phytanic acid levels are elevated. Sensory and motor NCS reveal reduced amplitudes, prolonged latencies, and slowed conduction velocities. Nerve biopsy demonstrates a loss of myelinated nerve fibers, with remaining axons often thinly myelinated and associated with onion bulb formation. Ref Harrison 20th edition page 3211