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: Pharmacology
A PATIENT WAS ON LITHIUM THERAPY FOR BIPOLAR DISORDER FOR 6MONTHS SHE KEPT THE FAST FOR FEW DAYS .SHE PRESENTED WITH S3 , COARSE TREMORS , CONFUSION AND WEAKNESS OF LIMBS. WHICH OF THE FOLLOWING SHOULD BE DONE NEXT TO ASSESS HER CONDITION ? 
Select and explain. 
A. S.ELECTROLYTES, B. ECG, C. MRI, D. S.LITHIUM LEVELS
D: S.LITHIUM LEVELS
Answer: D: S.LITHIUM LEVELS 
Explanation: REF : KD TRIPATHI 8TH ED