Q26.Which of the following is not a vitamin?
(A) Biotin
(B) Choline
(C) Histidine
(D) Nicotinamide
The correct answer is (C) Histidine. Histidine is one of the 20 standard proteinogenic amino acids essential for protein synthesis, not classified as a vitamin despite occasional conditional dietary needs in specific cases.
Vitamins are organic compounds required in trace amounts that humans cannot synthesize adequately, whereas amino acids like histidine are building blocks of proteins.
Option Analysis
Biotin (A): Vitamin B7, a water-soluble B-complex vitamin essential as a coenzyme for carboxylase enzymes in fatty acid synthesis, gluconeogenesis, and amino acid metabolism.
Choline (B): Essential nutrient often grouped with B-vitamins; functions in acetylcholine synthesis, lipid metabolism, and cell membrane structure (phospholipids). Not officially a vitamin but recognized as conditionally essential.
Histidine (C): Non-essential amino acid (in adults) coded by DNA for protein synthesis; precursor to histamine. Synthesized by the body, disqualifying it as a vitamin.
Nicotinamide (D): The active form of vitamin B3 (niacin); key component of NAD+ and NADP+ coenzymes in redox reactions, energy metabolism, and DNA repair.
Histidine stands out as not a vitamin among these options, functioning as a protein-building amino acid rather than a micronutrient humans must obtain from diet. This distinction matters in nutrition and biochemistry exam contexts.
Nutrient Classification
Biotin enables carboxylation reactions as vitamin B7. Choline supports methylation and membranes. Nicotinamide powers energy metabolism as B3 form. Histidine builds proteins endogenously.
