This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of protein and amino acid, Subject name: Biochemistry
Creatine is synthesized from which of the following? 
Select and explain. 
A. Methionine, B. Histidine, C. Proline, D. Lysine
A: Methionine
Answer: A: Methionine 
Explanation: Creatine is synthesized from glycine, arginine, and methionine. clinical impoance: 1. Creatine kinase has 3 isoenzymes, which are elevated in the following diseases: CK-MM: Muscular dystrophy, acute kidney injury, autoimmune myositis, hypothyroidism, malignant hypehermia, and drug-induced myopathy CK-BB: Stroke, brain injury, and meningitis CK-MB: Myocardial infarction, and myocarditis 2. Normally, creatinine is rapidly removed from the blood and excreted in the urine. Hence, creatinine clearance is a sensitive indicator of kidney function. Reference: Harpers illustrated biochemistry 30th edition