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
All are causes of spastic paraplegia except: 
Select and explain. 
A. Subacute combined degeneration, B. Motor neuron disease, C. Acute demyelinating encephalopathy, D. Fluorosis
D: Fluorosis
Answer: D: Fluorosis 
Explanation: Ans: d (Fluorosis)Ref: Harrison, 17 ed. pg. 2595 Adam & Victor NeurologySACD ( B defeciency)* Subacute paraesthesia in hand & feet, spatic paraparesis, loss of posterior column sensations, optic atrophy, irritability/mental changes.Causes loss of ankle jerk with extensor plantar.Motor neuron disease* Usually presents as insidious onset assymetric weakness of distally in one of the limbs.* Progressive wasting and atrophy of the muscles with fasciculations* Exaggerated DTR with spastic weakness of the limbs in predominant corticospinal involvement* Spares cognitive functions, bowel & bladder, sensory system and ocular motility till late stages.ADEM: Clinical featuresEncephalitic form - fever, headache, confusion, convulsions, coma, decerebrate rigidity.Myelitic form -Partial or complete paraplegia/quadriplegiawith varying degree of bowel& bladder involvement.Flurosis: Even though there are case reports about fluorosis presenting as spastic paraparesis due to involvement of the spine it is very rare and hence the most possible answer here.