Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 751: What is the primary biochemical consequence of a deficiency in vitamin B12?

A. Impaired DNA synthesis (Correct Answer)
B. Decreased fatty acid synthesis
C. Reduced protein translation
D. Enhanced glycolysis

Explanation: ***Impaired DNA synthesis*** - Vitamin B12 is essential for the conversion of **methylmalonyl-CoA** to **succinyl-CoA** and the methylation of **homocysteine to methionine**. - The latter reaction, catalyzed by **methionine synthase**, is crucial for regenerating **tetrahydrofolate**, which is necessary for **purine and pyrimidine synthesis** (components of DNA). *Decreased fatty acid synthesis* - While vitamin B12 is involved in **fatty acid metabolism** (specifically odd-chain fatty acids), its primary and most significant biochemical consequence in deficiency is not decreased overall fatty acid synthesis. - **Malonyl-CoA** and **acetyl-CoA carboxylase** are key enzymes in fatty acid synthesis, not directly dependent on B12 for their primary activity. *Reduced protein translation* - Protein translation primarily relies on **ribosomes**, **tRNA**, **mRNA**, and amino acids, with its regulation largely independent of vitamin B12's direct enzymatic roles. - While prolonged deficiency can indirectly affect overall cellular function and thus protein synthesis, it's not the immediate primary biochemical consequence. *Enhanced glycolysis* - Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating ATP and NADH. - Vitamin B12 is not a direct cofactor or regulator in the glycolytic pathway; therefore, its deficiency would not directly enhance this process.

Question 752: What is the role of vitamin B12 in DNA synthesis and neurological function?

A. Important for vitamin C metabolism; deficiency causes scurvy.
B. Key in the metabolism of carbohydrates; deficiency leads to lactic acidosis.
C. Involved in the metabolism of folate; deficiency leads to megaloblastic anemia.
D. Essential for DNA synthesis through methionine synthase reaction; deficiency leads to neuropathy. (Correct Answer)

Explanation: ***Essential for DNA synthesis through methionine synthase reaction; deficiency leads to neuropathy*** - **Vitamin B12** (cobalamin) is a crucial coenzyme for **methionine synthase**, an enzyme vital for converting **homocysteine to methionine**. This reaction is critical for generating **tetrahydrofolate**, which is necessary for **purine and pyrimidine synthesis**, thus impacting **DNA synthesis**. - B12 is also involved in the metabolism of **methylmalonyl-CoA**, and its deficiency leads to an accumulation of **methylmalonic acid**, which is toxic to the nervous system and causes **neurological dysfunction** and demyelination (neuropathy). - This option correctly addresses **both** aspects mentioned in the question: DNA synthesis and neurological function. *Important for vitamin C metabolism; deficiency causes scurvy* - **Vitamin C** (ascorbic acid) is essential for **collagen synthesis** and acts as an antioxidant. - Its deficiency leads to **scurvy**, characterized by gum inflammation, poor wound healing, and musculoskeletal pain. - This describes vitamin C, not vitamin B12. *Key in the metabolism of carbohydrates; deficiency leads to lactic acidosis* - **Thiamine** (vitamin B1) plays a key role in **carbohydrate metabolism** as a coenzyme in pathways like the citric acid cycle and pentose phosphate pathway. - **Thiamine deficiency** can lead to **lactic acidosis** and conditions like beriberi and Wernicke-Korsakoff syndrome. - This describes thiamine (B1), not vitamin B12. *Involved in the metabolism of folate; deficiency leads to megaloblastic anemia* - While vitamin B12 and **folate** metabolism are interconnected, this option is **incomplete** as an answer to the question. - Vitamin B12 is needed to regenerate **tetrahydrofolate** (active folate) via methionine synthase, and B12 deficiency does cause **megaloblastic anemia** through functional folate deficiency. - However, this option fails to mention the **neurological component**, which is a unique and critical feature of B12 deficiency that distinguishes it from pure folate deficiency. - The question specifically asks about "DNA synthesis AND neurological function" - this option only addresses the hematological manifestation.

Question 753: A patient with recurrent kidney stones has elevated levels of oxalate. What is the role of vitamin B6 in managing this condition, and what dietary modification should be suggested?

A. Decrease vitamin B6 intake to increase glyoxylate production
B. Increase vitamin C intake to reduce oxalate absorption
C. Decrease vitamin C intake to increase oxalate excretion
D. Increase vitamin B6 through dietary sources to reduce oxalate production (Correct Answer)

Explanation: ***Increase vitamin B6 through dietary sources to reduce oxalate production*** - **Vitamin B6 (pyridoxine)** is a cofactor for the enzyme **alanine:glyoxylate aminotransferase (AGT)**, which converts **glyoxylate to glycine**. - Without sufficient B6, glyoxylate is converted to **oxalate**, leading to hyperoxaluria and an increased risk of **calcium oxalate kidney stones**. - Therefore, increasing B6 intake helps **reduce oxalate production** and prevent stone formation. *Decrease vitamin B6 intake to increase glyoxylate production* - Decreasing vitamin B6 intake would **hinder the conversion of glyoxylate to glycine**, thereby **increasing glyoxylate levels** and subsequently increasing **oxalate production**. - This action would worsen hyperoxaluria and the risk of **kidney stone formation**, directly contradicting the goal of management. *Increase vitamin C intake to reduce oxalate absorption* - **High doses of vitamin C (ascorbic acid)** can be metabolized to **oxalate** in the body, potentially increasing urinary oxalate excretion rather than reducing absorption. - While vitamin C does not directly reduce oxalate absorption, its excessive intake should be **avoided** in patients with hyperoxaluria due to its metabolic conversion to oxalate. *Decrease vitamin C intake to increase oxalate excretion* - Decreasing vitamin C intake might **modestly reduce the body's oxalate load** since vitamin C can be metabolized to oxalate, but it does not directly increase oxalate excretion. - The primary aim in hyperoxaluria is to **reduce oxalate production or absorption**, and focusing solely on increasing excretion by reducing vitamin C is not the most effective or primary strategy.

Question 754: A patient with muscle twitching and cramps is found to have hypocalcemia. Which vitamin deficiency could be contributing to these symptoms?

A. Vitamin D (Correct Answer)
B. Vitamin B12
C. Vitamin A
D. Vitamin E

Explanation: ***Vitamin D*** - **Vitamin D** plays a crucial role in **calcium absorption** in the intestines and maintaining normal serum calcium levels through its interaction with parathyroid hormone. - A deficiency in vitamin D can lead to **hypocalcemia**, which often manifests with **neuromuscular excitability** symptoms such as muscle twitching (fasciculations) and cramps. *Vitamin B12* - **Vitamin B12** is essential for neurological function and blood cell formation; its deficiency primarily causes **megaloblastic anemia** and **neuropathy** (e.g., paresthesias, gait disturbances). - It does not directly regulate calcium metabolism, and its deficiency is not a primary cause of **hypocalcemia**. *Vitamin A* - **Vitamin A** is important for vision, immune function, and epithelial cell differentiation. - Its deficiency can lead to **night blindness**, xerophthalmia, and impaired immunity, but it does not directly impact **calcium homeostasis**. *Vitamin E* - **Vitamin E** is a powerful antioxidant that protects cell membranes from oxidative damage. - Deficiency is rare and can cause **neurological symptoms** like ataxia and peripheral neuropathy, but it is not linked to **calcium metabolism** or the direct cause of hypocalcemia.

Question 755: What is the recommended daily allowance of vitamin B1 (Thiamine) per 100 kilocalories of energy intake?

A. 0.05 mg (Correct Answer)
B. 0.5 mg
C. 5.0 mg
D. 1.0 mg

Explanation: ***0.05 mg*** - The recommended daily allowance (RDA) of **thiamine (vitamin B1)** is typically correlated with **energy intake**, at approximately 0.05 mg per 100 kilocalories. - This ensures sufficient thiamine for **carbohydrate metabolism**, a crucial process for energy production. *0.5 mg* - This value is significantly higher than the recommended daily allowance linked to energy intake and could lead to unnecessary supplementation if applied per 100 kcal. - While thiamine has a wide safety margin, such a high value per 100 kcal is not aligned with standard nutritional guidelines. *5.0 mg* - This is an excessively high amount of thiamine per 100 kilocalories, far exceeding physiological requirements. - Such a dosage per 100 kcal would likely only be considered in specific clinical scenarios of severe thiamine deficiency, not as a general daily recommendation. *1.0 mg* - While 1.0 mg might be a component of an overall daily thiamine intake for an adult, it is disproportionately high when considered per 100 kilocalories of energy. - A person consuming 2000 kcal would then need 20 mg of thiamine, which is significantly above the general RDA for most adults.

Question 756: Which of the following vitamins is significantly synthesised in gut by intestinal flora?

A. B12
B. B6
C. Folate
D. Biotin (Correct Answer)

Explanation: ***Biotin*** - **Biotin (Vitamin B7)** can be synthesized by **intestinal flora** in humans, contributing to the body's overall supply. - While dietary intake is the primary source, **gut bacteria** play a significant role in its endogenous production. *Folate* - Although some bacteria can synthesize **folate**, the human body primarily relies on **dietary intake** and absorption in the small intestine. - The contribution of gut flora to human folate requirements is generally considered **minor** and insufficient to meet daily needs. *B12* - **Vitamin B12 (cobalamin)** is exclusively synthesized by **microorganisms**, but in humans, this synthesis occurs mainly in the colon, where absorption is limited. - Humans must obtain **B12** from **animal products** or fortified foods, as intestinal flora synthesis does not contribute significantly to usable amounts. *B6* - **Vitamin B6 (pyridoxine)** is primarily obtained through **dietary sources** such as meat, fish, and vegetables. - The synthesis of **B6** by intestinal flora is **negligible** and does not contribute meaningfully to human requirements.

Question 757: Which of the following statements about NAD and NADP is true?

A. Niacin is a major precursor for NAD and NADP synthesis (Correct Answer)
B. High leucine intake directly inhibits NAD synthesis
C. All isoforms of malic enzyme exclusively use NAD as cofactor
D. NAD and NADP deficiency symptoms include dermatitis and dementia

Explanation: ***Niacin is a major precursor for NAD and NADP synthesis*** - **Niacin**, also known as vitamin B3, is a crucial nutrient required for the biosynthesis of both **nicotinamide adenine dinucleotide (NAD)** and **nicotinamide adenine dinucleotide phosphate (NADP)**. - The body converts niacin into these coenzymes, essential for numerous **redox reactions** in metabolism. *High leucine intake directly inhibits NAD synthesis* - While high intake of certain **amino acids** can affect metabolic pathways, **leucine** specifically is known to inhibit the activity of **kynureninase**, an enzyme in the kynurenine pathway which is involved in niacin synthesis from tryptophan. - However, it does not directly inhibit the synthesis of NAD itself but rather impacts a pathway leading to one of its precursors, which is distinct from a direct inhibition of NAD synthesis. *All isoforms of malic enzyme exclusively use NAD as cofactor* - **Malic enzyme** is a family of enzymes that catalyze the oxidative decarboxylation of malate to pyruvate. - Different isoforms exist: **ME1 (cytosolic)** uses **NADP**, while **ME2 and ME3 (mitochondrial)** use **NAD**; thus, not all isoforms exclusively use NAD. *NAD and NADP deficiency symptoms include dermatitis and dementia* - The symptoms of **dermatitis** and **dementia**, along with diarrhea, are characteristic of **pellagra**, a disease caused by severe **niacin (vitamin B3) deficiency**. - While NAD and NADP are derived from niacin, the deficiency symptoms are attributed to the lack of the precursor molecule, niacin, rather than a direct deficiency of NAD and NADP as separate entities.

Question 758: Which vitamin is commonly referred to as the 'antistress' vitamin?

A. Vitamin B1
B. Vitamin B2
C. Vitamin B3
D. Vitamin B5 (Correct Answer)

Explanation: ***Vitamin B5*** - **Vitamin B5** (pantothenic acid) is often called the "antistress" vitamin due to its crucial role in **adrenal gland function** and the production of **stress hormones**. - It participates in the synthesis of **coenzyme A (CoA)**, essential for energy metabolism and the production of neurotransmitters. *Vitamin B1* - **Vitamin B1 (thiamine)** is vital in **carbohydrate metabolism** and nerve function, but it is not directly associated with stress management. - Deficiency leads to **beriberi**, affecting the cardiovascular and nervous systems, rather than stress adaptation. *Vitamin B2* - **Vitamin B2 (riboflavin)** is a component of **flavin coenzymes**, critical for energy production and cellular growth, but it is not primarily known for antistress effects. - Its deficiency can lead to **ariboflavinosis**, affecting skin and mucous membranes. *Vitamin B3* - **Vitamin B3 (niacin)** plays a role in energy metabolism and DNA repair, primarily known for preventing **pellagra**. - While it can influence neurotransmitters, it is not primarily referred to as the "antistress" vitamin in the same way B5 is.

Question 759: Vitamin B12 is required for all of the following processes except?

A. Synthesis of thymidine
B. Conversion of homocysteine to cysteine (Correct Answer)
C. Isomerization of methylmalonyl-CoA
D. Conversion of homocysteine to methionine

Explanation: ***Conversion of homocysteine to cysteine*** - The conversion of **homocysteine to cysteine** involves two enzymes: **cystathionine beta-synthase (CBS)** and **cystathionine gamma-lyase (CTH)**. These enzymes require **vitamin B6** (pyridoxal phosphate) as a cofactor, not vitamin B12. - This pathway is part of the **transsulfuration pathway**, which is critical for homocysteine metabolism and cysteine synthesis. *Synthesis of thymidine* - **Vitamin B12**, as a cofactor for **methionine synthase**, is indirectly involved in the synthesis of **tetrahydrofolate**, which is essential for the production of **thymidylate** (a precursor to thymidine) from dUDP. - Deficiency in vitamin B12 can lead to a "folate trap," impairing DNA synthesis and causing **megaloblastic anemia**. *Isomerization of methylmalonyl-CoA* - **Methylmalonyl-CoA mutase**, an enzyme directly requiring **vitamin B12** (specifically, 5'-deoxyadenosylcobalamin), catalyzes the isomerization of **methylmalonyl-CoA to succinyl-CoA**. - This reaction is crucial for the metabolism of odd-chain fatty acids and certain amino acids, and its impairment in B12 deficiency leads to the accumulation of **methylmalonic acid**. *Conversion of homocysteine to methionine* - The enzyme **methionine synthase** (also known as homocysteine methyltransferase) catalyzes the conversion of **homocysteine to methionine**, using **vitamin B12** (specifically, methylcobalamin) as a coenzyme. - This reaction also requires **methylenetetrahydrofolate reductase** (MTHFR) and is critical for both homocysteine metabolism and the regeneration of **tetrahydrofolate**.

Question 760: Which of the following is not a good source of vitamin C?

A. Amla
B. Lime
C. Guava
D. Egg (Correct Answer)

Explanation: ***Egg*** - Eggs are a good source of **protein**, **choline**, **vitamins B12 and D**, but contain **negligible amounts of vitamin C**. - Vitamin C is primarily found in **fruits and vegetables**. *Amla* - **Amla** (Indian gooseberry) is an exceptionally rich source of **vitamin C**, often used in traditional medicine for its high content. - It contains significantly more vitamin C than many other fruits, making it a powerful antioxidant. *Lime* - **Lime**, like other citrus fruits, is an excellent source of **vitamin C**, contributing to immune function and collagen synthesis. - A single lime can provide a substantial portion of the recommended daily intake of vitamin C. *Guava* - **Guava** is remarkably rich in **vitamin C**, often containing several times the amount found in oranges. - It also provides fiber and other antioxidants, making it a highly nutritious fruit.

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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