Answer- A. Serum potassiumHypokalemia can cause flaccid ascending paralysis, which is a differential diagnosis for GBS. It can also cause periodic paralysis. Hypocalcemia and hypomagnesemia usually cause tetany and hypercalcemia won't cause ascending paralysis.A low serum potassium level during an attack, excluding secondary causes, establishes the diagnosis of Hypokalemic Periodic Paralysis.