This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Asthma, Subject name: Pharmacology
Therapeutic drug monitoring of Theophylline, when used clinically in Asthma, is because? 
Select and explain. 
A. Narrow therapeutic margin, B. Idiosyncrasy, C. Nephrotoxic, D. Decreases Histamine
A: Narrow therapeutic margin
Answer: A: Narrow therapeutic margin 
Explanation: ANSWER: (A) Narrow therapeutic marginREF: KDT 6th edition page 35, 221, Katzung 12" edition page 339Theophylline should be used only when therapeutic drug monitoring available because it has narrow therapeutic window.Therapeutic drug monitoring (TDM) is particularly useful in the following situations:Drugs with low safety margin-digoxin, anticonvulsants, antiarrhythmics, theophylline,aminoglycoside antibiotics, lithium, tricyclic antidepressants.If individual variations are large; antidepressants; lithium.Potentially toxic drugs used in the presence of renal failure; aminoglycoside antibiotics, vancomycm.In case of poisoning.In case of failure of response without any apparent reason; antimicrobials.To check patient compliance; psychopharmacological agents.