This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pathology
A pt of BPD on lithilhium for 6months after fasting for a day presents with seizure coarse tremors confusion and weakness of limbs Whats the most accurate to diagnosis her condition 
Select and explain. 
A. Serum Electrolyte, B. s.Lithium, C. S .creatinine, D. ECG
B: s.Lithium
Answer: B: s.Lithium 
Explanation: ref : harissons 21st ed