**Question:** An essential amino acid in man is

A. Histidine
B. Arginine
C. Isoleucine
D. Valine

**Correct Answer:** D. Valine

**Core Concept:**
An essential amino acid is a type of amino acid that cannot be synthesized by the human body and must be obtained from the diet. Adequate intake of essential amino acids is crucial for proper protein synthesis, growth, and maintenance of tissues in humans. There are nine essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

**Why the Correct Answer is Right:**
Valine is an essential amino acid because the human body cannot synthesize it internally. It plays a vital role in protein synthesis, energy production, and maintaining cellular function. Valine is included in the branched-chain amino acid (BCAA) group, along with isoleucine and leucine, which share similar physiological functions.

**Why Each Wrong Option is Incorrect:**
A. Histidine is not essential as the body can synthesize it in sufficient amounts. Although histidine is involved in several physiological processes, it is not considered essential in the context of this question.
B. Arginine is an amino acid, but it is not essential in humans. The body can synthesize arginine, and its role in protein synthesis and nitric oxide production is not sufficient to classify it as essential.
C. Isoleucine is part of the branched-chain amino acid (BCAA) group but is not essential as the body can synthesize it internally.

**Clinical Pearl:**
Ensuring a balanced intake of essential amino acids, including valine, helps support overall health and wellbeing. Adequate intake of branched-chain amino acids can contribute to muscle growth, repair, and maintenance, as well as energy production and cellular function. A diet lacking in these essential amino acids may lead to impaired protein synthesis and other physiological deficits.