This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
A patient presents with ascending muscle weakness for 2 days. On examination, the limb is flaccid. What investigation should be done first? 
Select and explain. 
A. Serum potassium, B. Serum creatinine, C. Serum magnesium, D. Serum calcium
A: Serum potassium
Answer: A: Serum potassium 
Explanation: 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.