This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pharmacology, Subject name: Pharmacology
Therapeutic Drug Monitoring (TDM) involves measurement of plasma concentrations of drugs to find whether the drug levels are within the therapeutic range or not. For TDM to be clinically useful the following criteria should be fulfilled: 
Select and explain. 
A. There should be good relationship between plasma concentration and drug dosage, B. The relationship between plasma drug concentration and therapeutic response and /or toxicity should be poor, C. When pharmacodynamic tolerance is suspected, D. When the clinical response cannot be easily monitored
D: When the clinical response cannot be easily monitored
Answer: D: When the clinical response cannot be easily monitored 
Explanation: Therapeutic drug monitoring (TDM) is useful in the following situations: 1. Drugs with low safety margin-digoxin, anticonvulsants, antiarrhythmics, theophylline, aminoglycosides, lithium, tricyclic antidepressants, vancomycin In case of failure of response without any apparent reason-antimicrobials, To check patient compliance-psychopharmacological agents Monitoring of plasma concentration is of no value for1. Drugs whose response is easily measurable antihypeensives, hypoglycaemics, diuretics, oral anticoagulants(Warfarin), general anesthetics.2. Drugs activated in the body-levodopa.3. 'Hit and run drugs' (whose effect lasts much longer than the drug itself)-reserpine, guanethidine, MAO inhibitors, omeprazole.4. Drugs with irreversible action-organophosphate anticholinesterases, phenoxybenzamine. Ref ; KD Tripathi pharmacology 7th edition (page no; 34)