Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 801: Which of the following is a Fat Soluble vitamin?

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

Explanation: ***Vitamin A*** - **Vitamin A** is one of the four essential **fat-soluble vitamins** (A, D, E, K), meaning it dissolves in fat and is stored in the body's fatty tissues and liver. - Its **absorption** relies on dietary fats and bile salts, distinguishing it from water-soluble vitamins. *Thiamine* - **Thiamine** (Vitamin B1) is a **water-soluble vitamin** that plays a crucial role in metabolism, converting carbohydrates into energy. - It is **not stored** in large amounts in the body and needs to be replenished regularly through diet. *Niacin* - **Niacin** (Vitamin B3) is a **water-soluble vitamin** vital for cellular metabolism, involving in DNA repair and steroid hormone synthesis. - It is readily excreted in urine when consumed in excess and does not accumulate in fatty tissues. *Riboflavin* - **Riboflavin** (Vitamin B2) is a **water-soluble vitamin** essential for energy production, cellular function, and the metabolism of fats, drugs, and steroids. - Like other B vitamins, it has limited storage in the body and is not dissolved in fat.

Question 802: 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 803: Pantothenic acid is needed for donating the following moiety?

A. Carboxyl
B. Hydroxyl
C. Amino
D. Acetyl group (Correct Answer)

Explanation: ***Acetyl group*** - **Pantothenic acid** (vitamin B5) is a crucial component of **coenzyme A (CoA)**, which plays a central role in transferring **acetyl groups**. - This transfer is vital in numerous metabolic pathways, including the **Krebs cycle**, fatty acid synthesis, and fatty acid oxidation. *Carboxyl* - The transfer of **carboxyl groups** is typically mediated by a different coenzyme, **biotin** (vitamin B7), not pantothenic acid. - Biotin-dependent enzymes are involved in carboxylation reactions like those in gluconeogenesis and fatty acid synthesis. *Hydroxyl* - While hydroxyl groups are common in organic chemistry, pantothenic acid's primary role is not the donation of isolated **hydroxyl groups**. - Hydroxyl groups are often donated or accepted during redox reactions or hydrolysis, often by specific enzymes directly. *Amino* - The transfer of **amino groups** is primarily associated with **pyridoxal phosphate (PLP)**, the active form of vitamin B6. - PLP-dependent enzymes, such as **transaminases**, are critical for amino acid metabolism.

Question 804: Pyridoxine deficiency leads to altered metabolism of?

A. Phenylalanine
B. Methionine
C. Tyrosine
D. Tryptophan (Correct Answer)

Explanation: ***Tryptophan*** - **Pyridoxine (vitamin B6)** is a critical coenzyme in the metabolism of **tryptophan**, particularly in its conversion to **niacin** and serotonin. - A deficiency leads to an accumulation of abnormal tryptophan metabolites, such as **xanthurenic acid**, which can be excreted in the urine. *Phenylalanine* - The metabolism of phenylalanine involves its conversion to tyrosine, a process catalyzed by **phenylalanine hydroxylase**, which does not directly require pyridoxine. - Deficiencies in phenylalanine metabolism often point to issues like **phenylketonuria**. *Methionine* - Methionine metabolism involves a cycle that generates **S-adenosylmethionine (SAM)** and then homocysteine. - While vitamin B6 is involved in the transsulfuration pathway (converting homocysteine to cysteine), its primary direct impact on methionine metabolism itself is less pronounced than on tryptophan. *Tyrosine* - Tyrosine is synthesized from phenylalanine and is a precursor for **catecholamines** and thyroid hormones. - Its metabolism does not directly rely on pyridoxine as a coenzyme in the main initial steps.

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: 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 809: 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 810: Which of the following is NOT a manifestation of vitamin E deficiency?

A. Hemolysis
B. Muscle weakness
C. Neurological Involvement
D. Thrombocytopenia (Correct Answer)

Explanation: ***Thrombocytopenia*** - **Thrombocytopenia** (low platelet count) is NOT typically associated with vitamin E deficiency. - Vitamin E primarily acts as an **antioxidant** protecting cell membranes, and its deficiency impacts **red blood cells** and **neurological function**, not platelet production or count. *Hemolysis* - Vitamin E is a crucial **antioxidant** that protects red blood cell membranes from **oxidative damage**. - Deficiency leads to increased RBC membrane fragility and premature destruction, resulting in **hemolytic anemia**. - This is a **classic manifestation** of vitamin E deficiency, especially in premature infants and patients with fat malabsorption. *Muscle weakness* - Vitamin E deficiency causes **myopathy** and **muscle weakness** due to oxidative damage to muscle tissue. - Patients may present with **proximal muscle weakness** and elevated **creatine kinase** levels. - This is part of the neuromuscular syndrome associated with chronic vitamin E deficiency. *Neurological Involvement* - Vitamin E deficiency is well-known to cause **neurological symptoms**, particularly affecting the **spinocerebellar tracts** and **dorsal columns**. - This leads to **spinocerebellar ataxia**, **peripheral neuropathy**, **loss of deep tendon reflexes**, and **posterior column dysfunction** (impaired proprioception and vibration sense). - **Retinopathy** may also occur due to oxidative damage to neuronal membranes.

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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