Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 811: 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**.

Question 812: Which of the following vitamins is not directly involved in energy metabolism?

A. Vitamin B1
B. Vitamin B3
C. Vitamin B7
D. Vitamin B12 (Correct Answer)

Explanation: ***Vitamin B12*** - Vitamin B12 (cobalamin) is considered the **least directly involved** in the major energy-producing pathways compared to the other B vitamins listed. - While B12 does play a role in energy metabolism through the conversion of **methylmalonyl-CoA to succinyl-CoA** (which enters the TCA cycle), this pathway primarily processes **odd-chain fatty acids**, **propionate**, and certain **amino acids** (valine, isoleucine, methionine, threonine). - This represents a **minor and specialized route** into energy metabolism, rather than involvement in the major pathways of glucose and even-chain fatty acid oxidation. - B12's primary clinical importance relates to **DNA synthesis**, **red blood cell formation**, and **nervous system function** rather than direct ATP generation from major fuel sources. *Vitamin B1* - As **thiamine pyrophosphate (TPP)**, Vitamin B1 is a crucial coenzyme directly involved in **carbohydrate metabolism**. - Essential for the **pyruvate dehydrogenase complex** (linking glycolysis to the TCA cycle) and **α-ketoglutarate dehydrogenase** (within the TCA cycle). - Directly produces **acetyl-CoA** for ATP generation from glucose. *Vitamin B3* - **Niacin**, in its coenzyme forms **NAD+** and **NADP+**, is central to all major energy pathways. - Essential for **glycolysis**, the **TCA cycle**, **fatty acid oxidation**, and the **electron transport chain** where it functions as an electron carrier. - Directly involved in ATP production from all major fuel sources. *Vitamin B7* - **Biotin** is a coenzyme for **carboxylase enzymes** involved in carbon dioxide transfer reactions. - Critical for **pyruvate carboxylase** (gluconeogenesis and TCA cycle anaplerosis), **acetyl-CoA carboxylase** (fatty acid synthesis), and amino acid catabolism. - Directly involved in maintaining metabolic pathways for energy substrate utilization.

Question 813: Vitamin D is maximum in which of the following?

A. Cod liver oil (Correct Answer)
B. Fortified milk
C. Egg yolks
D. Fatty fish

Explanation: **Cod liver oil** - **Cod liver oil** is the richest dietary source of vitamin D, containing approximately 10,000-30,000 IU per 100g - Historically used to prevent and treat rickets due to its exceptionally high concentration of this fat-soluble vitamin - Natural extraction from cod liver results in significantly higher vitamin D levels than any other dietary source *Fortified milk* - While commonly fortified with vitamin D, milk typically contains only 40-100 IU per 100ml - Fortification provides adequate daily intake but nowhere near the concentration found in cod liver oil - Useful for population-level prevention but not the maximum source *Egg yolks* - Contains relatively small amounts of vitamin D (approximately 20-50 IU per yolk) - Amount varies depending on the hen's diet and sunlight exposure - Considered a minor source compared to concentrated supplements or fish oils *Fatty fish* - Excellent natural sources of vitamin D (salmon, mackerel contain 400-1,000 IU per 100g) - While significantly higher than fortified foods, still substantially lower than cod liver oil - The concentrated liver oil extract has much higher vitamin D density than fish muscle tissue

Question 814: Which vitamin deficiency causes methylmalonic aciduria?

A. Vitamin B1
B. Vitamin B12 (Correct Answer)
C. Vitamin B6
D. Vitamin B5

Explanation: ***Vitamin B12*** - **Vitamin B12** is a crucial cofactor for the enzyme **methylmalonyl-CoA mutase**, which converts **methylmalonyl-CoA to succinyl-CoA**. - A deficiency in Vitamin B12 directly impairs this conversion, leading to an accumulation of **methylmalonic acid** (methylmalonic aciduria) and **methylmalonyl-CoA**. *Vitamin B1* - **Vitamin B1 (Thiamine)** deficiencies are associated with conditions like **Beriberi** and **Wernicke-Korsakoff syndrome**, which affect carbohydrate metabolism and neuronal function. - It does not play a direct role in the metabolism of methylmalonic acid. *Vitamin B6* - **Vitamin B6 (Pyridoxine)** is involved in **amino acid metabolism** and neurotransmitter synthesis. - Its deficiency can lead to neurological symptoms and microcytic anemia, but not methylmalonic aciduria. *Vitamin B5* - **Vitamin B5 (Pantothenic acid)** is a component of **coenzyme A (CoA)**, essential for fatty acid synthesis and degradation, and the citric acid cycle. - While important for metabolism, its deficiency does not directly cause methylmalonic aciduria.

Question 815: Which form of tetrahydrofolate (THF) serves as the central intermediate in the one-carbon pool and is most directly involved in thymidine synthesis?

A. None of the options
B. Methylene THF (Correct Answer)
C. Methyl THF
D. Formyl THF

Explanation: ***Methylene THF*** - **N5,N10-Methylene tetrahydrofolate** is the direct **one-carbon donor** for the methylation of deoxyuridylate (dUMP) to deoxythymidylate (dTMP) by the enzyme **thymidylate synthase**, which is crucial for **DNA synthesis**. - During this reaction, **methylene THF** is oxidized to **dihydrofolate (DHF)**, requiring reduction by **dihydrofolate reductase** (DHFR) to regenerate THF. *Methyl THF* - **N5-Methyl tetrahydrofolate** is primarily involved in the **methionine synthase** reaction to convert **homocysteine to methionine**, requiring **vitamin B12**. - It is not directly involved in thymidine synthesis but plays a role in remethylation pathways and the **folate trap** when B12 is deficient. *None of the options* - This option is incorrect because **methylene THF** is indeed a direct participant in thymidine synthesis, serving as the critical one-carbon donor. - The roles of various THF forms in one-carbon metabolism are well-defined, and one of them is specifically responsible for this function. *Formyl THF* - **N10-Formyl tetrahydrofolate** is essential for the synthesis of **purine nucleotides** by donating formyl groups at two steps in the purine ring formation. - While important for nucleic acid synthesis, it does not directly contribute the methyl group needed for thymidine synthesis from dUMP.

Question 816: Which reaction requires Vitamin B1?

A. None of the options
B. Oxidative decarboxylation (Correct Answer)
C. Carboxylation
D. Transamination

Explanation: ***Oxidative decarboxylation*** - Vitamin B1, in its active form **thiamine pyrophosphate (TPP)**, is a crucial coenzyme for enzymes catalyzing **oxidative decarboxylation** reactions. - Key examples include the **pyruvate dehydrogenase complex** and **alpha-ketoglutarate dehydrogenase complex**, essential for cellular respiration and the citric acid cycle. *Transamination* - This type of reaction, involving the transfer of an **amino group**, primarily requires **pyridoxal phosphate (PLP)**, the active form of **Vitamin B6**. - It is vital for amino acid metabolism but does not utilize Vitamin B1. *Carboxylation* - **Carboxylation** reactions, which add a carboxyl group to a substrate, typically require **biotin** (Vitamin B7) as a coenzyme. - Examples include pyruvate carboxylase and acetyl-CoA carboxylase, which are not dependent on Vitamin B1. *None of the options* - As **oxidative decarboxylation** specifically requires Vitamin B1, this option is incorrect. - The other listed reactions depend on different vitamins as coenzymes.

Question 817: Which vitamin deficiency is associated with decreased RBC transketolase activity?

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

Explanation: ***Thiamine deficiency*** - **Transketolase** is an enzyme in the **pentose phosphate pathway** that requires **thiamine pyrophosphate (TPP)**, the active form of thiamine, as a cofactor. - A deficiency in thiamine directly impairs transketolase activity, which can be measured in red blood cells to assess thiamine status. *Folic acid deficiency* - **Folic acid** is essential for **DNA synthesis** and **amino acid metabolism**, and its deficiency leads to **megaloblastic anemia**. - It is not directly involved as a cofactor for the transketolase enzyme. *Vitamin B12 deficiency* - **Vitamin B12** (cobalamin) is crucial for **DNA synthesis** and the proper functioning of the nervous system, with its deficiency also causing **megaloblastic anemia** and neurological symptoms. - It does not serve as a cofactor for the transketolase enzyme. *Vitamin C deficiency* - **Vitamin C** (ascorbic acid) is vital for **collagen synthesis**, **immune function**, and as an antioxidant, and its deficiency causes **scurvy**. - It has no known direct co-enzymatic role with transketolase.

Question 818: The most important regulator of serum 1,25(OH)2 vitamin D concentration is:

A. Calcium levels in serum
B. Magnesium levels in serum
C. Parathyroid hormone (Correct Answer)
D. 25-hydroxyvitamin D in serum

Explanation: ***Parathyroid hormone*** - **Parathyroid hormone (PTH)** directly stimulates the **kidney's 1-alpha hydroxylase** enzyme, which converts **25(OH)D** to its active form, **1,25(OH)2D (calcitriol)**. - This regulation is critical for maintaining **calcium and phosphate homeostasis**, with PTH levels increasing when serum calcium is low, thereby boosting 1,25(OH)2D production. *Calcium levels in serum* - While **low serum calcium** indirectly stimulates **PTH** release, which then regulates 1,25(OH)2 vitamin D, calcium itself is not the direct or most important regulator. - The direct regulatory action on the conversion enzyme is mediated by PTH. *Magnesium levels in serum* - **Magnesium** plays a cofactor role in various enzymatic reactions, including those involving vitamin D metabolism, but it is not a direct or primary regulator of **1,25(OH)2 vitamin D concentration**. - Severe **hypomagnesemia** can sometimes impair PTH secretion and action, indirectly affecting vitamin D, but this is a secondary effect. *25-hydroxyvitamin D in serum* - **25-hydroxyvitamin D** is the precursor to **1,25(OH)2 vitamin D**, and its availability limits the maximum potential production of the active form. - However, the *rate* of conversion into the active form and thus the *concentration* of 1,25(OH)2D is primarily dictated by PTH, not the precursor itself.

Question 819: Hydroxyl radicals are destroyed in the body by?

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

Explanation: ***Vitamin E*** - **Vitamin E** (α-tocopherol) is the **primary fat-soluble antioxidant** that directly scavenges and neutralizes **hydroxyl radicals (•OH)** and other reactive oxygen species in lipid membranes. - It is the **most important chain-breaking antioxidant** in biological membranes, protecting polyunsaturated fatty acids from lipid peroxidation initiated by hydroxyl radicals. - Vitamin E donates hydrogen atoms to free radicals, converting them into less reactive forms and preventing oxidative damage to cellular structures. *Vitamin D* - **Vitamin D** is crucial for **calcium homeostasis** and **bone health**, and plays a role in immune function. - It is **not a direct antioxidant** and does not have a significant role in scavenging hydroxyl radicals. *Vitamin C* - **Vitamin C** (ascorbate) is a **water-soluble antioxidant** that primarily works in aqueous environments. - While it can scavenge some free radicals, its main antioxidant role is **regenerating oxidized Vitamin E** back to its active form, thus indirectly supporting the antioxidant defense system. - It is more effective against superoxide and other water-soluble reactive oxygen species than hydroxyl radicals. *Vitamin A* - **Vitamin A** (retinoids and carotenoids) is important for **vision**, **immune function**, and **cell differentiation**. - Some forms like **beta-carotene** can act as antioxidants in lipid environments at low oxygen tensions, but they are **not the primary defense** against hydroxyl radicals.

Question 820: Thiamine deficiency is known to occur in all of the following except.

A. Food faddist
B. Chronic heart failure
C. Homocystinemia (Correct Answer)
D. Chronic alcoholic

Explanation: **Homocystinemia** - **Homocystinemia** is a metabolic disorder characterized by elevated levels of **homocysteine**, usually due to deficiencies in enzymes involved in methionine metabolism, not thiamine. - This condition is typically associated with deficiencies in vitamins **B6, B12, or folate**, which are cofactors for enzymes like cystathionine beta-synthase and methionine synthase. *Food faddist* - Individuals with highly restrictive or **unbalanced diets** may exclude thiamine-rich foods, leading to deficiency. - This dietary pattern can result in an inadequate intake of essential nutrients, including **thiamine**. *Chronic heart failure* - Patients with **chronic heart failure** are at increased risk for **thiamine deficiency** due to diuretic use, which can increase urinary excretion of water-soluble vitamins. - **Reduced absorption** in the gastrointestinal tract and **poor dietary intake** are also contributing factors in these patients. *Chronic alcoholic* - **Chronic alcohol abuse** is a major cause of thiamine deficiency due to **poor nutrition**, **impaired absorption** of thiamine, and **increased metabolic demand**. - Alcohol can directly interfere with thiamine transport and phosphorylation, leading to **Wernicke-Korsakoff syndrome** and other neurological complications.

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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