Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 791: Which vitamin deficiency is most commonly associated with chronic alcohol consumption and may require supplementation in alcohol use disorders?

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

Explanation: ***Thiamine*** - **Thiamine** (Vitamin B1) deficiency is extremely common in chronic alcoholics due to **poor dietary intake**, impaired absorption, and increased excretion. - Deficiency can lead to serious neurological complications like **Wernicke-Korsakoff syndrome**, characterized by confusion, ataxia, and ophthalmoplegia. *Pyridoxine* - **Pyridoxine** (Vitamin B6) deficiency can occur in alcoholics, but it is less common and less clinically significant than thiamine deficiency. - While it can manifest as peripheral neuropathy or **sideroblastic anemia**, it is not the most common or critical deficiency in this population. *Riboflavin* - **Riboflavin** (Vitamin B2) deficiency can also be seen in chronic alcoholics due to poor nutrition. - Symptoms like **cheilosis** and **angular stomatitis** are mild compared to the severe neurological consequences of thiamine deficiency. *Niacin* - **Niacin** (Vitamin B3) deficiency, known as **pellagra**, is rare in developed countries but can occur in severe malnutrition, including in some alcoholics. - Pellagra presents with the "3 Ds": **dermatitis**, **diarrhea**, and **dementia**, but it is generally less prevalent than thiamine deficiency in alcohol use disorders.

Question 792: What vitamin is significantly absent in breast milk?

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

Explanation: ***Vitamin K*** - Breast milk contains **critically low amounts** of **vitamin K** (approximately 2-5 μg/L), making it the **most significantly absent** vitamin in breast milk. - Newborns are at high risk for **Vitamin K deficiency bleeding (VKDB)**, which can be life-threatening. - This deficiency is so critical that **prophylactic vitamin K injection** is routinely administered to **all newborns** at birth to prevent hemorrhagic disease. - Vitamin K is essential for the synthesis of **clotting factors II, VII, IX, and X** in the liver. *Vitamin D* - While breast milk does contain **some vitamin D**, the levels are often **insufficient** to meet the infant's daily requirements (contains ~25-78 IU/L; infant needs 400 IU/day). - However, it is **present in measurable amounts**, unlike vitamin K which is nearly absent. - Infants are recommended to receive **vitamin D supplementation** starting soon after birth, especially if the mother has low vitamin D levels or limited **sun exposure**. *Vitamin C* - **Breast milk** contains adequate amounts of **vitamin C** (40-50 mg/L), which is sufficient for the infant's needs. - Maternal diet typically provides enough vitamin C to ensure its presence in breast milk. *Vitamin A* - Breast milk is a **good source** of **vitamin A**, particularly in the form of **beta-carotene** and retinol. - Vitamin A levels in breast milk are adequate for infant **vision development** and **immune function**.

Question 793: What is the daily requirement of vitamin K?

A. 5-10 mg/kg
B. 0.5-1 mg/kg
C. 1-2 mcg/kg (Correct Answer)
D. 10-15 mcg/kg

Explanation: ***1-2 mcg/kg*** - The daily requirement of **vitamin K** for adults is approximately **1-2 mcg/kg body weight** (or about 90-120 mcg/day for average adults). - This amount is sufficient for **γ-carboxylation** of clotting factors II, VII, IX, and X, as well as proteins C and S. - The **Adequate Intake (AI)** set by dietary guidelines supports normal coagulation and bone health at these levels. *0.5-1 mg/kg* - This represents a **500-1000 fold excess** over the actual requirement (mg instead of mcg). - This is a **unit error** - the requirement is in **micrograms (mcg)**, not milligrams (mg). - Such high doses would be **pharmacological** rather than physiological, though vitamin K has relatively low toxicity. *10-15 mcg/kg* - This is approximately **10 times higher** than the actual daily requirement. - While not toxic, this amount is **unnecessarily high** for maintaining normal hemostasis. - Typical dietary intake and physiological needs are much lower. *5-10 mg/kg* - This represents an extremely **excessive amount** (5000-10000 times the requirement). - Another example of a **unit confusion** (mg vs mcg). - Such doses have no physiological benefit and are not used clinically except in specific therapeutic situations (e.g., warfarin reversal).

Question 794: 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 795: Which of the following requires vitamin B12?

A. Conversion of serine to lysine
B. Conversion of serine to glycine
C. Conversion of glutamine to glutamate
D. Conversion of homocysteine to methionine (Correct Answer)

Explanation: ***Homocysteine to methionine*** - The conversion of **homocysteine to methionine** is catalyzed by **methionine synthase**, an enzyme that requires **vitamin B12** (cobalamin) as a cofactor. - **Vitamin B12** facilitates the transfer of a methyl group from **methyltetrahydrofolate** to homocysteine, forming methionine. *Conversion of serine to lysine* - The metabolism of **serine to lysine** involves multiple steps and different enzymes, but it does not directly require **vitamin B12**. - Lysine is an **essential amino acid** and is primarily obtained from dietary sources or synthesized through complex pathways. *Conversion of serine to glycine* - The conversion of **serine to glycine** is catalyzed by **serine hydroxymethyltransferase**, which requires **tetrahydrofolate (THF)** as a cofactor, not vitamin B12. - This reaction generates **5,10-methylenetetrahydrofolate**, an important one-carbon donor. *Conversion of glutamine to glutamate* - The conversion of **glutamine to glutamate** is primarily catalyzed by **glutaminase**, an enzyme that does not require **vitamin B12**. - This reaction involves the removal of an **ammonia group** from glutamine to form glutamate.

Question 796: 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 797: The cofactor vitamin B12 is required for the following conversion:

A. Dopamine to Norepinephrine
B. Propionyl CoA to methyl malonyl CoA
C. Methyl malonyl CoA to succinyl CoA (Correct Answer)
D. Homocysteine to cysteine

Explanation: ***Methyl malonyl CoA to succinyl CoA*** - **Vitamin B12**, in its active form **adenosylcobalamin**, is a crucial cofactor for the enzyme **methylmalonyl-CoA mutase**, which catalyzes the isomerization of **methylmalonyl-CoA to succinyl-CoA**. - This conversion is vital for the metabolism of **odd-chain fatty acids** and certain **amino acids**, allowing their entry into the **Krebs cycle**. *Dopamine to Norepinephrine* - This conversion is catalyzed by **dopamine beta-hydroxylase**, which requires **vitamin C** (ascorbate) and **copper** as cofactors, not vitamin B12. - It is a key step in the synthesis of **catecholamines** within the nervous system. *Propionyl CoA to methyl malonyl CoA* - This conversion is catalyzed by **propionyl-CoA carboxylase** and requires **biotin** as a cofactor, not vitamin B12. - This reaction is the first step in the metabolic pathway that leads to succinyl-CoA from odd-chain fatty acids. *Homocysteine to cysteine* - This conversion occurs via the **transsulfuration pathway** and requires **vitamin B6** (pyridoxal phosphate) as a cofactor, not vitamin B12. - The enzymes involved are **cystathionine β-synthase** and **cystathionine γ-lyase**, both B6-dependent. - Vitamin B12 is involved in the **remethylation** of homocysteine to methionine (not in transsulfuration to cysteine).

Question 798: Which vitamin is involved in one-carbon transfer reactions?

A. Folic acid (Correct Answer)
B. Pantothenic acid
C. Niacin
D. Thiamine

Explanation: ***Folic acid*** - **Folic acid (Vitamin B9)** is a crucial coenzyme in the form of **tetrahydrofolate (THF)**, which acts as a carrier of **one-carbon units**. - These one-carbon units are essential for various metabolic pathways, including the synthesis of **purines**, **thymidylate**, and the metabolism of several **amino acids**. *Pantothenic acid* - **Pantothenic acid (Vitamin B5)** is a precursor to **Coenzyme A (CoA)**, which plays a central role in fatty acid metabolism and the **Krebs cycle**, not one-carbon transfers. - CoA is involved in transferring **acetyl groups**, not one-carbon units. *Niacin* - **Niacin (Vitamin B3)** is a component of **NAD+** and **NADP+**, which are vital coenzymes in **redox reactions** (electron transfer), not one-carbon metabolism. - It functions primarily in **energy metabolism** as an electron carrier. *Thiamine* - **Thiamine (Vitamin B1)** is a coenzyme for **dehydrogenase reactions** and **transketolase** in the **pentose phosphate pathway**, which are involved in carbohydrate metabolism. - It does not directly participate in one-carbon transfer reactions.

Question 799: Which vitamin serves as the primary fat-soluble antioxidant protecting cell membranes in the human body?

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

Explanation: ***Vitamin E*** - **Vitamin E** is the primary **fat-soluble antioxidant** in the human body, specifically protecting **cell membranes** from **oxidative damage**. - It neutralizes **free radicals** that can harm lipid-rich structures like cell membranes. *Vitamin K* - **Vitamin K** is primarily involved in **blood clotting** (coagulation) and **bone metabolism**. - It does not function as a primary antioxidant in cell membranes. *Vitamin C* - **Vitamin C** is a **water-soluble antioxidant** that works in aqueous environments, such as the cytosol and blood plasma. - While a potent antioxidant, it does not specifically protect fat-soluble cell membranes. *Vitamin A* - **Vitamin A** (retinol) is essential for **vision**, **immune function**, and **cell growth**. - While it has some antioxidant properties, it is not the primary fat-soluble antioxidant protecting cell membranes in the same way as Vitamin E.

Question 800: 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.

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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