Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 811: 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.

Question 812: 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 813: 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 814: 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 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: 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.

Question 820: 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.

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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