Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 801: Which of the following vitamins does not participate in the conversion of pyruvate to acetyl CoA?

A. Thiamine
B. Riboflavin
C. Biotin (Correct Answer)
D. Niacin

Explanation: ***Biotin*** - **Biotin** is a cofactor for **carboxylases** (e.g., pyruvate carboxylase), which are involved in reactions such as **gluconeogenesis** and fatty acid synthesis, not the oxidative decarboxylation of pyruvate to acetyl CoA. - While it plays a crucial role in metabolism, its specific function does not include the pyruvate dehydrogenase complex. *Thiamine* - **Thiamine pyrophosphate (TPP)**, derived from thiamine (vitamin B1), is an essential coenzyme for the **pyruvate dehydrogenase complex**, specifically for the E1 subunit, which decarboxylates pyruvate. - A deficiency in thiamine would impair the conversion of pyruvate to acetyl CoA, leading to lactic acid accumulation. *Niacin* - **Niacin** (vitamin B3) is a precursor for **NAD+ (nicotinamide adenine dinucleotide)**, which acts as an electron acceptor in the pyruvate dehydrogenase complex, specifically with the E3 subunit (dihydrolipoyl dehydrogenase). - NAD+ is reduced to **NADH** during the oxidation of dihydrolipoyl. *Riboflavin* - **Riboflavin** (vitamin B2) is a precursor for **FAD (flavin adenine dinucleotide)**, which is covalently bound to the E3 subunit (dihydrolipoyl dehydrogenase) of the pyruvate dehydrogenase complex. - FAD cycles between oxidized (FAD) and reduced (FADH2) forms as it transfers electrons.

Question 802: Which vitamin is considered the most potent antioxidant?

A. Vit A
B. Vit K
C. Vit E (Correct Answer)
D. Vit C

Explanation: ***Vit E*** - **Vitamin E** is a **lipid-soluble antioxidant** that primarily protects cell membranes from **oxidative damage** by scavenging free radicals. - Its ability to interrupt **lipid peroxidation** makes it highly effective in protecting tissues rich in polyunsaturated fatty acids, such as cell membranes. *Vit A* - **Vitamin A**, particularly in its carotenoid forms like **beta-carotene**, is an antioxidant, but its primary role is in **vision** and **immune function**. - While it can quench **singlet oxygen** and trap free radicals, it is generally considered less potent than vitamin E in protecting against lipid peroxidation. *Vit K* - **Vitamin K** is crucial for **blood coagulation** and **bone metabolism**, but it does not have significant antioxidant properties. - Its primary biological functions are unrelated to scavenging **free radicals** or preventing oxidative stress. *Vit C* - **Vitamin C** is a potent **water-soluble antioxidant** that works in aqueous environments, such as the cytoplasm and extracellular fluid. - While it can neutralize **reactive oxygen species** and regenerate other antioxidants like vitamin E, its solubility limits its direct activity in protecting lipid membranes, making vitamin E more potent in that specific context.

Question 803: What is the coenzyme form of pyridoxine?

A. ADP
B. NAD
C. PLP (Correct Answer)
D. FAD

Explanation: ***PLP*** - **Pyridoxal phosphate (PLP)** is the active coenzyme form of **pyridoxine (vitamin B6)**. - It plays a crucial role in numerous metabolic reactions, particularly those involving **amino acid metabolism**. *ADP* - **Adenosine diphosphate (ADP)** is an important molecule in energy transfer, particularly in the formation of **ATP (adenosine triphosphate)**. - It is not a coenzyme form of any vitamin, but rather a **nucleotide**. *NAD* - **Nicotinamide adenine dinucleotide (NAD)** is a coenzyme derived from **niacin (vitamin B3)**. - It functions as an electron carrier in **redox reactions** and is vital for cellular respiration. *FAD* - **Flavin adenine dinucleotide (FAD)** is a coenzyme derived from **riboflavin (vitamin B2)**. - It also serves as an electron carrier in **redox reactions**, particularly in the electron transport chain.

Question 804: In which condition is the utilization of pyruvate in tissues decreased?

A. Pernicious anemia
B. Scurvy
C. Beriberi (Correct Answer)
D. Pellagra

Explanation: ***Beriberi*** - Beriberi is caused by **thiamine (vitamin B1) deficiency**, which is a crucial cofactor for the **pyruvate dehydrogenase complex (PDH)**. - A dysfunctional PDH enzyme leads to a decreased conversion of **pyruvate to acetyl-CoA**, thus **decreasing pyruvate utilization** and causing its accumulation. *Pernicious anemia* - This condition is caused by a deficiency in **vitamin B12 (cobalamin)**, typically due to a lack of intrinsic factor, leading to **megaloblastic anemia**. - While vitamin B12 is essential for various metabolic pathways, it does not directly impair the utilization of **pyruvate** by PDH. *Scurvy* - Scurvy results from **vitamin C (ascorbic acid) deficiency**, which is essential for collagen synthesis and acts as an antioxidant. - Vitamin C deficiency does not directly impact the activity of the **pyruvate dehydrogenase complex** or the utilization of pyruvate. *Pellagra* - Pellagra is caused by a deficiency in **niacin (vitamin B3)**, or its precursor, tryptophan. - Niacin is a component of **NAD+ and NADP+**, which are crucial coenzymes in many metabolic reactions, but its deficiency does not primarily manifest as decreased **pyruvate utilization**.

Question 805: Vitamin A intoxication causes injury to

A. Lysosomal membranes (Correct Answer)
B. Protein synthesis
C. Structural support
D. Energy metabolism

Explanation: ***Lysosomal membranes*** - **Vitamin A toxicity** destabilizes **lysosomal membranes**, causing release of hydrolytic enzymes - Release of **lysosomal enzymes** leads to uncontrolled enzymatic digestion and cellular injury - This is the **primary mechanism** of tissue damage in hypervitaminosis A *Protein synthesis* - While vitamin A affects **gene expression** and **cellular differentiation**, toxicity does not primarily target protein synthesis - Acute toxic effects are mediated through **membrane damage** rather than ribosomal inhibition *Structural support* - Vitamin A is important for **bone development**, but toxicity does not directly damage structural components - Structural damage occurs **secondarily** due to lysosomal enzyme release and membrane instability *Energy metabolism* - Vitamin A toxicity does not directly disrupt **mitochondrial function** or **ATP production** - Mechanism differs from metabolic poisons that inhibit the **electron transport chain** or **glycolysis**

Question 806: What is the naturally occurring form of vitamin D synthesized in the human skin?

A. 25 hydroxy cholecalciferol
B. 1,25 dihydroxy cholecalciferol
C. 7-dehydrocholesterol
D. Cholecalciferol (Vitamin D3) (Correct Answer)

Explanation: ***Cholecalciferol (Vitamin D3)*** - This is the form of vitamin D synthesized in the skin when exposed to **ultraviolet B (UVB) radiation**. - **7-dehydrocholesterol** in the skin is converted to cholecalciferol by UVB light. *7-dehydrocholesterol* - This is the **precursor** molecule in the skin that is converted into vitamin D3 upon exposure to UVB light. - It is not the final naturally occurring form of vitamin D, but rather the **substrate** for its synthesis. *25 hydroxy cholecalciferol* - This is the **storage form** of vitamin D, also known as **calcidiol**, produced in the liver from cholecalciferol. - It circulates in the blood and is used to assess an individual's vitamin D status. *1,25 dihydroxy cholecalciferol* - This is the **active form** of vitamin D, also known as **calcitriol**, primarily synthesized in the kidneys from 25-hydroxycholecalciferol. - It plays a crucial role in **calcium and phosphate homeostasis** by acting as a hormone.

Question 807: Biotin is needed in what process?

A. Fatty acid synthesis (Correct Answer)
B. Krebs cycle
C. Urea cycle
D. Pyruvate dehydrogenase

Explanation: ***Fatty acid synthesis*** - **Biotin** is a required cofactor for **acetyl-CoA carboxylase**, the enzyme catalyzing the rate-limiting step in **fatty acid synthesis** - It helps in the carboxylation of **acetyl-CoA** to **malonyl-CoA**, which is crucial for elongating the fatty acid chain - **Note**: Biotin is a cofactor for multiple carboxylase enzymes (acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, and methylcrotonyl-CoA carboxylase), but among the given options, fatty acid synthesis is the most directly associated process *Krebs cycle* - The Krebs cycle (citric acid cycle) primarily involves enzymes that utilize cofactors like **NAD+**, **FAD**, and **thiamine pyrophosphate (TPP)**, not biotin - Its main function is to oxidize acetyl-CoA to CO₂, producing ATP precursors *Urea cycle* - The urea cycle mainly uses enzymes that require **ATP**, **N-acetylglutamate**, and various amino acids, but **biotin is not a direct cofactor** for any of its core enzymes - While pyruvate carboxylase (a biotin-dependent enzyme) supports anaplerotic reactions, it is not part of the urea cycle itself - The function of the urea cycle is to convert toxic ammonia into urea for excretion *Pyruvate dehydrogenase* - The **pyruvate dehydrogenase complex** requires several cofactors, including **thiamine pyrophosphate (TPP)**, **lipoic acid**, **FAD**, **NAD+**, and **coenzyme A (CoA)**, but not biotin - It links glycolysis to the Krebs cycle by converting pyruvate to acetyl-CoA

Question 808: Which of the following minerals is a component of glutathione peroxidase, a major antioxidant enzyme?

A. Copper
B. Selenium (Correct Answer)
C. Manganese
D. Zinc

Explanation: ***Correct: Selenium*** - **Selenium** is an essential component of **glutathione peroxidase (GPx)**, one of the body's most important antioxidant enzyme systems - It is incorporated as **selenocysteine** at the active site of the enzyme - Glutathione peroxidase **neutralizes hydrogen peroxide and organic hydroperoxides**, protecting cells from oxidative damage - Selenium deficiency leads to **Keshan disease** (cardiomyopathy) due to increased oxidative stress *Incorrect: Copper* - Copper is a cofactor for **superoxide dismutase (Cu-Zn SOD)** and **ceruloplasmin**, which have antioxidant properties - However, copper is NOT a component of glutathione peroxidase *Incorrect: Zinc* - Zinc is a component of **superoxide dismutase (Cu-Zn SOD)** and helps prevent lipid peroxidation - However, zinc is NOT a component of glutathione peroxidase *Incorrect: Manganese* - Manganese is a cofactor for **manganese superoxide dismutase (Mn-SOD)** in mitochondria - However, manganese is NOT a component of glutathione peroxidase

Question 809: Deficiency of which vitamin during early pregnancy will result in neural tube defect?

A. Folic acid (Correct Answer)
B. Iron
C. Cyanocobalamine
D. Vitamin E

Explanation: ***Folic acid*** - **Folic acid** (vitamin B9) is crucial for **neural tube closure** during the first 28 days of embryonic development. - Deficiency leads to conditions like **spina bifida** and **anencephaly**. - **Periconceptional supplementation** with 400-800 mcg daily is recommended to prevent neural tube defects. *Iron* - **Iron** is essential for **hemoglobin synthesis** and fetal growth, but its deficiency does not directly cause neural tube defects. - Iron deficiency in pregnancy can lead to **maternal anemia** and premature birth. *Cyanocobalamine* - **Cyanocobalamine** (vitamin B12) is involved in DNA synthesis and neurological function, but its direct link to neural tube defects is not as strong as folic acid. - Severe B12 deficiency can cause **megaloblastic anemia** and neurological issues. *Vitamin E* - **Vitamin E** is a fat-soluble antioxidant that protects cell membranes from oxidative damage. - While important for overall health during pregnancy, it does not play a specific role in neural tube closure. - Deficiency is rare and does not cause neural tube defects.

Question 810: Which of the following statements about folic acid and vitamin B12 is NOT true?

A. Vegetarians have vitamin B12 deficit
B. Deficiency of both causes megaloblastic anemia
C. Recommended daily allowances for Folic acid is 1000 micro grams (Correct Answer)
D. Both are required for DNA synthesis

Explanation: ***Recommended daily allowances for Folic acid is 1000 micro grams*** - The **recommended daily allowance (RDA)** for folic acid in adults is typically around **400 micrograms (mcg)**, not 1000 mcg. Pregnant women may require a higher intake of 600 mcg. - An intake of 1000 mcg (1 mg) is closer to the **tolerable upper intake level (UL)** for folic acid, above which there is a risk of masking a vitamin B12 deficiency. *Both are required for DNA synthesis* - Both **folic acid** (as tetrahydrofolate) and **vitamin B12** (as methylcobalamin) are essential coenzymes in the **one-carbon metabolism pathway**. - This pathway is crucial for the synthesis of **purines and pyrimidines**, which are the building blocks of **DNA**. *Vegetarians have vitamin B12 deficit* - **Vitamin B12 (cobalamin)** is found almost exclusively in **animal products** (meat, fish, eggs, dairy). - Therefore, individuals following a strict **vegetarian or vegan diet** are at a significant risk of developing a vitamin B12 deficiency if they do not consume fortified foods or supplements. *Deficiency of both causes megaloblastic anemia* - Both **folic acid deficiency** and **vitamin B12 deficiency** impair DNA synthesis, which leads to arrested maturation of red blood cell precursors in the bone marrow. - This results in the production of **large, immature red blood cells** known as **megaloblasts**, clinically manifesting as **megaloblastic anemia**.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free