Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 721: What is the major source of cyanocobalamin (vitamin B12)?

A. Sunlight
B. Plant-only diet
C. Animal-only diet (Correct Answer)
D. None of the options

Explanation: ***Animal-only diet*** - **Cyanocobalamin (vitamin B12)** is primarily found in **animal products** such as meat, fish, poultry, eggs, and dairy. - It is synthesized by microorganisms and accumulates in animal tissues, making animal-derived foods the most reliable dietary source for humans. *Sunlight* - **Sunlight** is the primary source of **vitamin D** synthesis in the skin, not vitamin B12. - Exposure to UV radiation from the sun does not contribute to the production or uptake of cyanocobalamin. *Plant-only diet* - A **plant-only diet (vegan diet)** is deficient in naturally occurring **vitamin B12**, as plants do not synthesize this vitamin. - Vegans often require **fortified foods** or **supplements** to meet their B12 needs. *None of the options* - This option is incorrect because an **animal-only diet** is identified as a major source of cyanocobalamin. - The question has a correct answer among the provided choices.

Question 722: Most abundant form of pro-vitamin A is:-

A. Retinol
B. Beta carotene (Correct Answer)
C. Alpha-carotene
D. Retinaldehyde

Explanation: ***Beta carotene*** - **Beta-carotene** is the most widely distributed and abundant form of **provitamin A** in plants. - It yields two molecules of **retinal** upon enzymatic cleavage, which can then be converted to **retinol**. *Retinol* - **Retinol** is the active form of **vitamin A** and is not a pro-vitamin. - It is derived from pro-vitamin A carotenoids or ingested directly as preformed vitamin A. *Alpha-carotene* - **Alpha-carotene** is another form of **provitamin A**, but it is **less abundant** than beta-carotene. - It yields only one molecule of **retinal** because of its asymmetric structure. *Retinaldehyde* - **Retinaldehyde** (retinal) is an intermediate metabolite in the **vitamin A pathway**, converted from provitamin A or retinol. - It is not considered a pro-vitamin but rather a direct precursor to other active forms of vitamin A, such as **retinol** and **retinoic acid**.

Question 723: A child was fed on a staple diet of maize for a long time. Which of the following vitamin may get deficient in his body?

A. Thiamine (Vitamin B1)
B. Vitamin B6 (Pyridoxine)
C. Cobalamin (Vitamin B12)
D. Niacin (Vitamin B3) (Correct Answer)

Explanation: ***Niacin (Vitamin B3)*** - Maize is deficient in tryptophan (an amino acid that can be converted to **niacin**) and contains niacin in a bound form (**niacytin**) that is not bioavailable. - A staple diet of maize without adequate supplementation can lead to **pellagra**, characterized by the classic triad of dermatitis, diarrhea, and dementia (3 D's). - This is particularly common in populations relying heavily on untreated maize as a staple food. *Thiamine (Vitamin B1)* - While polishing rice can remove thiamine, maize itself is not primarily associated with **thiamine deficiency** as a staple. - **Beriberi** (thiamine deficiency) presents with neurological and cardiovascular symptoms (wet and dry beriberi), distinct from pellagra. *Vitamin B6 (Pyridoxine)* - Deficiency of **pyridoxine** is uncommon with maize-based diets unless there are other contributing factors like drug interactions (e.g., isoniazid). - Symptoms include peripheral neuropathy, seborrheic dermatitis, and sideroblastic anemia, which are not directly linked to a maize staple diet. *Cobalamin (Vitamin B12)* - **Vitamin B12** is found primarily in animal products, so a vegetarian or vegan diet poses a risk for deficiency, not specifically a maize-based diet. - Deficiency leads to megaloblastic anemia and neurological damage (subacute combined degeneration), unrelated to maize's nutritional profile.

Question 724: Which coenzyme is required by 6-phosphogluconate dehydrogenase?

A. FAD
B. NADP (Correct Answer)
C. FMN
D. NAD

Explanation: ***Correct NADP*** - **6-phosphogluconate dehydrogenase** is a key enzyme in the **oxidative phase of the pentose phosphate pathway**, which generates **NADPH**. - **NADP+** acts as an electron acceptor, getting reduced to **NADPH**, which is crucial for reductive biosynthesis and **antioxidant defense**. *Incorrect FAD* - **Flavin adenine dinucleotide (FAD)** is a derivative of **riboflavin (vitamin B2)** and is typically involved in reactions transferring two electrons, often found in **dehydrogenases** that form carbon-carbon double bonds. - Enzymes like **succinate dehydrogenase** (in the TCA cycle) utilize **FAD**, which is not the case for 6-phosphogluconate dehydrogenase. *Incorrect FMN* - **Flavin mononucleotide (FMN)**, also derived from **riboflavin**, is primarily found in **flavoproteins** and acts as a prosthetic group in various **redox reactions**. - It is a component of **Complex I (NADH dehydrogenase)** in the electron transport chain, but not involved in the pentose phosphate pathway's 6-phosphogluconate dehydrogenase reaction. *Incorrect NAD* - **Nicotinamide adenine dinucleotide (NAD+)** is a coenzyme primarily involved in catabolic reactions where it accepts electrons and is reduced to **NADH**, typically in glycolysis, the TCA cycle, and fatty acid oxidation. - While structurally similar to NADP+, **NAD+** is involved in ATP production, whereas **NADP+** is used in anabolic processes.

Question 725: Female, on maize as a staple diet. History of diarrhoea and lesion in the neck region. This is due to deficiency of which of the following?

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

Explanation: ***Niacin*** * The constellation of **diarrhea**, **dermatitis** (neck lesion, often a "Casal's necklace"), and the exclusive reliance on a **maize staple diet** is highly characteristic of **pellagra**, which is caused by a **niacin (vitamin B3) deficiency**. * Maize contains niacin in a bound form (niacytin) that is largely **unbioavailable** unless treated with alkali (e.g., historical limewater soaking in some cultures). *Pyridoxine* * Deficiency of **pyridoxine (vitamin B6)** can lead to symptoms such as **skin inflammation (seborrheic dermatitis)**, **glossitis**, **cheilosis**, and neurological issues like **peripheral neuropathy** or **seizures**. * It does not typically present with the classic **diarrhea** and **dermatitis** pattern associated with pellagra in the context of a maize-based diet. *Thiamine* * **Thiamine (vitamin B1) deficiency** is associated with **beriberi**, characterized by cardiovascular symptoms (**wet beriberi** with edema and heart failure) or neurological symptoms (**dry beriberi** with neuropathy and muscle wasting). * It does not cause the specific dermatological lesion on the neck or the triad of symptoms (dermatitis, diarrhea, and dementia) seen with niacin deficiency. *Zinc* * **Zinc deficiency** can manifest as **dermatitis (acrodermatitis enteropathica)**, **diarrhea**, **impaired immune function**, and **poor wound healing** or **growth retardation**. * While it can cause skin lesions and diarrhea, the distinct **photosensitive dermatosis** on the neck and the strong association with a maize staple diet specifically point away from zinc deficiency and towards pellagra.

Question 726: Vitamin B12 is found maximum in:

A. Sunlight
B. Green leafy vegetables
C. Animal products (Correct Answer)
D. Roots and tubers

Explanation: ***Animal products*** - **Vitamin B12**, or cobalamin, is primarily synthesized by bacteria and accumulates in animal tissues, making **meat, fish, poultry, eggs, and dairy** the richest dietary sources. - Humans generally cannot synthesize vitamin B12 and must obtain it from dietary sources, with **animal-derived foods** being the most concentrated and bioavailable forms. *Sunlight* - **Sunlight** is primarily critical for the synthesis of **Vitamin D** in the skin, not Vitamin B12. - Exposure to sunlight has no direct role in the production or absorption of **Vitamin B12**. *Green leafy vegetables* - **Green leafy vegetables** are excellent sources of nutrients like **folate, Vitamin K, and iron**, but they are generally devoid of naturally occurring **Vitamin B12**. - While some fortified plant-based foods may contain Vitamin B12, naturally, these vegetables do not provide it. *Roots and tubers* - **Roots and tubers** like potatoes, carrots, and sweet potatoes are good sources of carbohydrates, fiber, and various vitamins and minerals such as **Vitamin C and potassium**. - They do not contain significant amounts of **Vitamin B12**, as they are plant-based foods.

Question 727: True statement about vitamin K is?

A. Vitamin K is needed for action of clotting factor 8
B. Vitamin K deficiency leads to DVT
C. Vitamin K is water soluble
D. Vitamin K affects bone health by activating proteins that bind calcium (Correct Answer)

Explanation: ***Vitamin K affects bone health by activating proteins that bind calcium*** - Vitamin K plays a crucial role in **osteocalcin activation**, a protein essential for integrating calcium into the bone matrix. - This activation process involves **gamma-carboxylation of glutamic acid residues** on osteocalcin, allowing it to bind calcium and strengthen bone. *Vitamin K is needed for action of clotting factor 8* - Vitamin K is essential for the activation of **clotting factors II, VII, IX, and X**, as well as proteins C and S, through gamma-carboxylation. - **Factor VIII** (antihemophilic factor A) is not directly dependent on vitamin K for its activation. *Vitamin K deficiency leads to DVT* - Vitamin K deficiency primarily leads to **impaired blood clotting** and an increased risk of bleeding, not DVT. - DVT (deep vein thrombosis) is typically associated with **hypercoagulable states**, venous stasis, or endothelial injury. *Vitamin K is water soluble* - Vitamin K is a **fat-soluble vitamin**, meaning it is absorbed with fats in the diet and stored in the body's fatty tissues and liver. - Water-soluble vitamins include **B vitamins and vitamin C**, which are not stored to the same extent and are excreted in urine.

Question 728: Which of the following is the rich source of Vitamin B12?

A. Mango
B. Carrot
C. Meat (Correct Answer)
D. Spinach

Explanation: ***Meat*** - **Vitamin B12** is primarily found in **animal products** because it is synthesized by bacteria in the digestive tracts of animals. - **Meat**, especially red meat and liver, is an excellent source of this vitamin. *Mango* - Mangoes are a good source of **Vitamin C** and **Vitamin A**, but they contain negligible amounts of Vitamin B12. - As a fruit, mangos are a plant-based food and generally **do not contain B12**. *Carrot* - Carrots are rich in **beta-carotene** (a precursor to Vitamin A) and **fiber**, but they are not a source of Vitamin B12. - Being a vegetable, carrots are a **plant-based food** and lack Vitamin B12. *Spinach* - Spinach is known for its high content of **iron**, **folate**, and **Vitamin K**, but it does not contain Vitamin B12. - Like other plant-based foods, spinach **naturally lacks Vitamin B12**.

Question 729: Thiamine deficiency is assessed by:

A. Serum thiamine level
B. RBC thiamine levels
C. Erythrocyte transketolase activity (Correct Answer)
D. RBC Glutathione reductase

Explanation: ***Erythrocyte transketolase activity*** - This is the **most reliable functional assay** for thiamine deficiency, as thiamine pyrophosphate (TPP) is a crucial cofactor for the enzyme **transketolase**. - A significant increase in transketolase activity after the addition of TPP *in vitro* indicates a deficiency, demonstrating the enzyme's reliance on exogenous thiamine. *Serum thiamine level* - **Serum thiamine levels** can fluctuate and do not accurately reflect the body's thiamine stores or functional status, as most thiamine is intracellular. - This measurement may be normal even in cases of functional deficiency or tissue depletion. *RBC thiamine levels* - While red blood cell (RBC) thiamine levels provide a better estimate of tissue stores compared to serum levels, they are **still less sensitive** than direct functional assays like transketolase activity. - RBC thiamine measurements do not directly assess the functional impact of thiamine deficiency on metabolic pathways. *RBC Glutathione reductase* - **Glutathione reductase** activity is used to assess **riboflavin (vitamin B2) deficiency**, not thiamine. - Riboflavin in its coenzyme form, **flavin adenine dinucleotide (FAD)**, is a necessary cofactor for glutathione reductase.

Question 730: Vitamin K in its coenzyme form is regenerated by which enzyme?

A. Epoxide reductase (Correct Answer)
B. Glutathione reductase
C. Dihydrofolate reductase
D. Pyruvate carboxylase

Explanation: ***Epoxide reductase*** - **Vitamin K epoxide reductase (VKOR)** regenerates the active form of vitamin K, **hydroquinone**, from its inactive **epoxide metabolite**, which is essential for the function of **gamma-glutamyl carboxylase**. - This enzyme is the target of **warfarin**, an anticoagulant drug that inhibits its activity, thus reducing the synthesis of active **clotting factors**. *Glutathione reductase* - This enzyme is involved in maintaining the cellular redox state by reducing **oxidized glutathione (GSSG)** back to **reduced glutathione (GSH)**, using **NADPH** as a cofactor. - It plays a crucial role in protecting cells from **oxidative stress**, but it is not directly involved in vitamin K metabolism. *Dihydrofolate reductase* - **Dihydrofolate reductase (DHFR)** catalyzes the reduction of **dihydrofolate** to **tetrahydrofolate (THF)**, a critical cofactor for **one-carbon transfer reactions**, including purine and pyrimidine synthesis. - It is the target of **methotrexate**, a chemotherapy drug, and **trimethoprim**, an antibiotic, but has no role in vitamin K recycling. *Pyruvate carboxylase* - **Pyruvate carboxylase** is a mitochondrial enzyme that converts **pyruvate** to **oxaloacetate**, a crucial step in **gluconeogenesis** and for replenishing intermediates of the **citric acid cycle**. - It requires **biotin** as a cofactor and is not involved in vitamin K metabolism or redox cycling.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free