**Question:** Therapeutic drug monitoring of plasma concentration of antihypertensive drugs is not practiced because

A. lack of correlation between plasma concentration and clinical effect
B. inadequate evidence for setting therapeutic drug monitoring targets
C. limited drug-drug interactions
D. difficulty in determining the optimal therapeutic range for each drug

**Correct Answer:** D. difficulty in determining the optimal therapeutic range for each drug

**Core Concept:**
Therapeutic drug monitoring (TDM) is a process of measuring the plasma concentration of a drug to determine whether the administered dose is within an optimal therapeutic range. For antihypertensive drugs, the therapeutic range is crucial, as excessive concentrations may cause side effects, while suboptimal concentrations may not effectively control hypertension.

**Why the Correct Answer is Right:**
In the case of antihypertensive drugs, therapeutic drug monitoring is not widely practiced due to the challenge of determining an optimal therapeutic range for each drug. Each antihypertensive medication has a unique pharmacokinetic and pharmacodynamic profile, which influences its therapeutic range. For example, the therapeutic range for a calcium channel blocker like nifedipine may differ from that of a beta-blocker like atenolol. This variation leads to difficulties in establishing a universal therapeutic range for all antihypertensive drugs.

**Why Each Wrong Option is Incorrect:**

A. Correlation between plasma concentration and clinical effect: In some cases, the correlation between plasma concentration and clinical effect may be weak or non-existent. However, this option does not address the specific challenge of determining the optimal therapeutic range for each antihypertensive drug.

B. Inadequate evidence for setting therapeutic drug monitoring targets: While evidence supporting therapeutic drug monitoring exists for some drugs, the lack of evidence for all antihypertensive drugs makes this option less relevant as a primary reason for not practicing TDM universally.

C. Limited drug-drug interactions: This option focuses on a different aspect of therapeutic drug monitoring and does not address the difficulty of determining the optimal therapeutic range for individual antihypertensive drugs.

**Clinical Pearl:**
Therapeutic drug monitoring is essential for certain medications, such as immunosuppressants, where a narrow therapeutic range is crucial to avoid adverse effects or inadequate therapeutic response. For antihypertensive drugs, the complexity arises from the need to determine an optimal therapeutic range for each individual drug, considering its pharmacokinetic and pharmacodynamic properties. This personalized approach is challenging in the context of hypertension management, where a one-size-fits-all approach is often more practical.