Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 831: What is the role of vitamin K in the epoxide cycle?

A. Carboxylation of aspartate
B. Carboxylation of glutamate (Correct Answer)
C. Dehydrogenation of aspartate
D. Dehydrogenation of glutamate

Explanation: ***Carboxylation of glutamate*** - Vitamin K acts as a **cofactor** for the enzyme **γ-glutamyl carboxylase**, which catalyzes the post-translational carboxylation of specific **glutamate residues** in certain proteins. - This carboxylation is crucial for these proteins, particularly **blood clotting factors**, to bind calcium ions and become functionally active. *Carboxylation of aspartate* - **Aspartate** residues are not the direct targets for vitamin K-dependent carboxylation. - While carboxylation is a vital biochemical modification, it is specifically carried out on **glutamate residues** in a vitamin K-dependent manner. *Dehydrogenation of aspartate* - **Dehydrogenation** involves the removal of hydrogen atoms and is not directly related to the role of vitamin K in the epoxide cycle. - The epoxide cycle is primarily concerned with the **redox cycling** of vitamin K and its role in carboxylation, not dehydrogenation. *Dehydrogenation of glutamate* - Similar to aspartate, **dehydrogenation of glutamate** is not the primary function or mechanism associated with vitamin K in the epoxide cycle. - Vitamin K's key role is in the **reductive carboxylation** of glutamate residues.

Question 832: All of the following are water-soluble antioxidants except?

A. uric acid
B. Polyphenols
C. Carotenes (Correct Answer)
D. Ascorbate

Explanation: ***Carotenes*** - **Carotenes** are **lipid-soluble** antioxidants that are typically found in cell membranes and lipid components of the body. - They protect against **lipid peroxidation**, which is damage to cellular membranes caused by free radicals. *Ascorbate* - **Ascorbate** (Vitamin C) is a well-known **water-soluble** antioxidant that readily donates electrons to neutralize free radicals in aqueous environments. - It plays a crucial role in regenerating other antioxidants, such as **alpha-tocopherol (Vitamin E)**. *Uric acid* - **Uric acid** is a potent **water-soluble** antioxidant present in high concentrations in the plasma. - It scavenges various reactive oxygen species and reactive nitrogen species, especially in the **extracellular fluid**. *Polyphenols* - **Polyphenols** are a large group of **water-soluble** compounds found in plants that act as antioxidants. - They reduce oxidative stress by **scavenging free radicals**, chelating metal ions, and inhibiting enzymes involved in radical generation.

Question 833: Which vitamin is required for transfer of 1-carbon unit?

A. Niacin
B. Vitamin B12
C. Vitamin A
D. Folic acid (Correct Answer)

Explanation: ***Folic acid*** - **Folic acid** (vitamin B9) is essential for the transfer of **one-carbon units**, particularly as **tetrahydrofolate (THF)**. - These one-carbon units are critical in metabolic processes such as **DNA synthesis**, **amino acid metabolism**, and **neurotransmitter synthesis**. *Vitamin A* - **Vitamin A** (retinol) is primarily involved in **vision**, **immune function**, and **cell differentiation**. - It does not play a direct role in the transfer of one-carbon units. *Vitamin B12* - **Vitamin B12** (cobalamin) is involved in two main reactions: the conversion of **methylmalonyl-CoA to succinyl-CoA** and the transfer of a **methyl group from N5-methyl THF to homocysteine** to form methionine. - While it works with folate, it does not directly transfer one-carbon units in the same way as folic acid. *Niacin* - **Niacin** (vitamin B3) is a precursor to **NAD+ and NADP+**, which are crucial coenzymes in **redox reactions** and energy metabolism. - It is not involved in the transfer of one-carbon units.

Question 834: Anti-infective vitamin is

A. Vitamin D
B. Vitamin C
C. Vitamin B6
D. Vitamin A (Correct Answer)

Explanation: ***Vitamin A*** - Vitamin A is known as the **anti-infective vitamin** because it plays a crucial role in maintaining the integrity of **epithelial tissues**, which serve as the body's first line of defense against pathogens. - It also supports the proper functioning of the **immune system**, including the development and differentiation of lymphocytes and other immune cells. *Vitamin B6* - Vitamin B6 (pyridoxine) is essential for numerous **metabolic functions**, including amino acid, carbohydrate, and fat metabolism. - While it contributes to overall health, it is not primarily known as an anti-infective vitamin in the same way as Vitamin A. *Vitamin D* - Vitamin D is primarily associated with **bone health** due to its role in calcium and phosphate regulation. - It also has immunomodulatory effects, but its primary function is not anti-infective protection of epithelial barriers. *Vitamin C* - Vitamin C (ascorbic acid) is a powerful **antioxidant** and plays a vital role in **collagen synthesis** and immune function. - While important for immunity, its primary role is not maintaining epithelial integrity against infection to the extent of Vitamin A.

Question 835: Which dietary deficiency of a vitamin can cause pellagra?

A. Niacin (Correct Answer)
B. Vitamin A
C. Folate
D. Vitamin B12

Explanation: ***Niacin*** - **Pellagra** is classically described by the "4 D's": **dermatitis**, **diarrhea**, **dementia**, and eventually **death**, all of which are caused by a deficiency in **niacin (Vitamin B3)**. - Niacin is crucial for energy metabolism and DNA repair, and its deficiency affects rapidly dividing cells, especially in the skin, gastrointestinal tract, and nervous system. *Vitamin A* - Deficiency in **Vitamin A** primarily leads to **vision problems**, including **night blindness** and, in severe cases, **xerophthalmia** and irreversible blindness. - While it can impact immune function and epithelial integrity, it does not cause the characteristic triad of pellagra. *Folate* - A deficiency in **folate (Vitamin B9)** is primarily associated with **megaloblastic anemia**, which involves enlarged, immature red blood cells. - It does not cause the dermatological, gastrointestinal, or neurological symptoms seen in pellagra. *Vitamin B12* - **Vitamin B12 (cobalamin) deficiency** also causes **megaloblastic anemia**, but it is uniquely associated with **neurological symptoms** such as peripheral neuropathy, cognitive impairment, and subacute combined degeneration of the spinal cord. - While there is some neurological overlap, its clinical presentation is distinct from pellagra and does not involve the characteristic dermatitis or severe diarrhea.

Question 836: Gamma-glutamyl carboxypeptidase is linked with the absorption of which vitamin?

A. Riboflavin
B. Niacin
C. Folic acid (Correct Answer)
D. Pyridoxine

Explanation: ***Folic acid*** - **Gamma-glutamyl carboxypeptidase**, also known as **folylpolyglutamate hydrolase** or **conjugase**, is an enzyme crucial for the absorption of dietary folic acid. - It hydrolyzes the polyglutamyl forms of folate found in food into **monoglutamyl folate**, which can then be absorbed in the small intestine. *Riboflavin* - **Riboflavin (vitamin B2)** absorption does not directly involve gamma-glutamyl carboxypeptidase. - Its absorption primarily relies on **active transport mechanisms** and facilitated diffusion in the small intestine. *Niacin* - **Niacin (vitamin B3)** absorption is mainly through **passive diffusion** and, at lower concentrations, by carrier-mediated transport. - It is not dependent on the enzymatic activity of gamma-glutamyl carboxypeptidase. *Pyridoxine* - **Pyridoxine (vitamin B6)** is absorbed mainly by **passive diffusion** in the jejunum. - Its absorption does not involve gamma-glutamyl carboxypeptidase.

Question 837: What is the form of folate that accumulates in vitamin B12 deficiency due to the inhibition of methionine synthase?

A. Methyl-THFA (Correct Answer)
B. Formyl THFA
C. Methylene-THFA
D. Methenyl THFA

Explanation: ***Methyl-THFA*** - In **vitamin B12 deficiency**, the enzyme **methionine synthase** is inhibited, as it requires B12 as a cofactor. - This leads to the accumulation of **methyl-THFA (5-methyltetrahydrofolate)** because it cannot be converted back to tetrahydrofolate (THF), trapping it in this form—known as the "**methyl-trap hypothesis**." *Formyl THFA* - **Formyl THFA (10-formyltetrahydrofolate)** is a folate derivative involved in purine synthesis, not directly accumulated due to B12 deficiency. - Its levels are not specifically noted to increase in the context of methionine synthase inhibition. *Methylene-THFA* - **Methylene-THFA (5,10-methylenetetrahydrofolate)** is an important substrate for **thymidylate synthase** in DNA synthesis. - While its metabolism is linked to folate, it does not accumulate as the primary "methyl-trap" form in B12 deficiency. *Methenyl THFA* - **Methenyl THFA (5,10-methenyltetrahydrofolate)** is an intermediate in folate metabolism, interconverting with methylene-THFA and formyl-THFA. - Like other folate derivatives not directly involved in the inhibited methionine synthase step, it doesn't accumulate specifically in B12 deficiency.

Question 838: Maximum concentration of vitamin A is found in which organ?

A. Liver (Correct Answer)
B. Kidney
C. Lung
D. Heart

Explanation: ***Liver*** - The **liver** is the primary organ for **storage of vitamin A** (retinyl esters), accounting for 90% of the body's total vitamin A content. - **Hepatic stellate cells** within the liver are specialized for storing the majority of this fat-soluble vitamin. *Kidney* - The **kidney** plays a role in **vitamin D metabolism** and excretion, but not significant vitamin A storage. - While it helps regulate blood levels of various substances, it does not accumulate large quantities of vitamin A. *Lung* - The **lung** does not serve as a major storage site for **vitamin A**. - Its primary functions are related to **gas exchange**, not nutrient storage. *Heart* - The **heart** is responsible for **pumping blood** throughout the body and has minimal involvement in vitamin A storage. - It utilizes certain vitamins for its metabolic processes but does not act as a primary reservoir.

Question 839: Which of the following organs are involved in the synthesis and activation of vitamin D?

A. Liver and Skin
B. Skin and Kidney
C. Kidney and Liver
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - The **skin** synthesizes an inactive form of vitamin D (**cholecalciferol**) upon exposure to **UVB radiation**. - The **liver** performs the first hydroxylation step, converting **cholecalciferol** (vitamin D3) into **25-hydroxyvitamin D** (calcidiol). The **kidneys** then perform the final hydroxylation, converting **calcidiol** into the active form, **1,25-dihydroxyvitamin D** (calcitriol). *Liver and Skin* - While both the **liver** and **skin** play crucial roles in vitamin D metabolism, they do not encompass all necessary organs. - The **kidneys** are essential for the final activation step of vitamin D. *Skin and Kidney* - The **skin** synthesizes the precursor, and the **kidneys** perform the final activation step. - However, the **liver** is required for the initial hydroxylation of vitamin D. *Kidney and Liver* - The **kidney** is responsible for the final activation, and the **liver** for the initial hydroxylation. - This option misses the crucial role of the **skin** in the initial synthesis of vitamin D upon sun exposure.

Question 840: A patient is found to have elevated levels of methylmalonic acid. This finding suggests a deficiency in which vitamin?

A. Folate (important for DNA synthesis and red blood cell production).
B. Vitamin D (essential for calcium and phosphate homeostasis).
C. Vitamin B6 (involved in neurotransmitter synthesis and amino acid metabolism).
D. Vitamin B12 (required for methylmalonic acid metabolism). (Correct Answer)

Explanation: ***Vitamin B12 (required for methylmalonic acid metabolism).*** - An elevated level of **methylmalonic acid** (MMA) is a sensitive and specific indicator of **vitamin B12 deficiency** because vitamin B12 is a coenzyme for the enzyme **methylmalonyl-CoA mutase**, which converts methylmalonyl-CoA to succinyl-CoA. - Without sufficient **vitamin B12**, MMA accumulates and can be measured in serum or urine, leading to **neurological symptoms** and **megaloblastic anemia**. *Vitamin B6 (involved in neurotransmitter synthesis and amino acid metabolism).* - **Vitamin B6** (pyridoxine) is a cofactor for enzymes involved in amino acid metabolism, neurotransmitter synthesis, and **heme synthesis**, but it does not directly metabolize methylmalonic acid. - Deficiency in vitamin B6 can cause **sideroblastic anemia**, **neuropathy**, and **dermatitis**, not elevated MMA. *Folate (important for DNA synthesis and red blood cell production).* - **Folate** (vitamin B9) is crucial for DNA synthesis, red blood cell maturation, and amino acid metabolism, often leading to **megaloblastic anemia** when deficient. - Although folate deficiency can also cause **macrocytic anemia**, it does not lead to an accumulation of **methylmalonic acid**, distinguishing it from vitamin B12 deficiency. *Vitamin D (essential for calcium and phosphate homeostasis).* - **Vitamin D** is primarily involved in **calcium and phosphate homeostasis**, bone health, and immune function. - Deficiency in vitamin D can cause **rickets** in children and **osteomalacia** in adults, but it has no direct role in the metabolism of **methylmalonic acid**.

Practice by Chapter

Fat-Soluble Vitamins: A, D, E, K

Practice Questions

Vitamin A and Vision

Practice Questions

Vitamin D and Calcium Metabolism

Practice Questions

Vitamin E and Antioxidant Functions

Practice Questions

Vitamin K and Blood Coagulation

Practice Questions

Water-Soluble Vitamins: B Complex and C

Practice Questions

Thiamine (B1) and Pyruvate Dehydrogenase

Practice Questions

Riboflavin (B2) and Flavin Coenzymes

Practice Questions

Niacin and NAD/NADP

Practice Questions

Vitamin B6 and Transamination

Practice Questions

Folate and Vitamin B12 in One-Carbon Metabolism

Practice Questions

Vitamin C and Collagen Synthesis

Practice Questions

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