**Question:** Intravenous inducing agent with maximum plasma protein binding -

A. Midazolam
B. Thiopentone
C. Fentanyl
D. Propofol

**Correct Answer:** B. Thiopentone

**Core Concept:**
Pharmacokinetics refers to the study of the distribution, absorption, and elimination of drugs within the body. Plasma protein binding describes the extent to which a drug binds to proteins in the blood, affecting its pharmacological action, volume of distribution, and overall clearance. Highly bound drugs are less free to interact with their targets, such as receptors or enzymes, which can lead to reduced therapeutic effects or adverse reactions.

**Why the Correct Answer is Right:**
Thiopentone (phenytoin) is a barbiturate anesthetic with a high degree of plasma protein binding (approximately 99%). This means that the majority of the drug molecules are bound to plasma proteins, reducing their free fraction available to interact with its target (GABA receptors) and causing anesthesia.

**Why Each Wrong Option is Incorrect:**

A. Midazolam (midazolam) has a moderate plasma protein binding (approximately 90%). While still bound, this lower binding fraction allows for more free drug to interact with its target (benzodiazepine receptors) and results in shorter sedation and recovery times compared to thiopentone.

B. Thiopentone has a higher plasma protein binding than thiopentone. This choice is incorrect as thiopentone has the highest binding rate among the options.

C. Fentanyl is an opioid analgesic with a low plasma protein binding (approximately 10%). Its lower binding fraction allows for more free drug to interact with its target (mu receptors) and results in rapid onset of action and shorter duration of action compared to thiopentone.

D. Propofol is a highly lipophilic drug used for anesthesia induction. However, it has a moderate plasma protein binding (approximately 99%). In comparison to thiopentone, its binding rate is not significantly higher, but it differs in terms of its pharmacokinetic properties such as faster onset of action and shorter duration of action due to its lipophilic nature.

**Clinical Pearl:**
Plasma protein binding is an essential factor to consider when selecting an anesthetic agent as it influences drug distribution, efficacy, and side effects. A higher binding rate usually results in a slower onset, longer duration, and potentially more side effects due to reduced free drug fraction to reach its target site. On the other hand, a lower binding rate results in faster onset and shorter duration but may lead to reduced efficacy due to less drug reaching the target site.