Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 771: Which of the following is NOT a manifestation of vitamin E deficiency?

A. Posterior column abnormalities
B. Cerebellar ataxia
C. Hemolytic anemia
D. Autonomic dysfunction (Correct Answer)

Explanation: ***Autonomic dysfunction*** - **Autonomic dysfunction** is not typically associated with vitamin E deficiency. Instead, it is commonly seen in conditions like **diabetes mellitus**, Parkinson's disease, or certain inherited neuropathies. - Vitamin E primarily acts as an **antioxidant** and is crucial for neurological and red blood cell health. *Hemolytic anemia* - Vitamin E is an **antioxidant** that protects red blood cell membranes from **oxidative damage**. - Its deficiency can lead to increased fragility and **hemolysis** of red blood cells, resulting in hemolytic anemia, particularly in premature infants. *Posterior column abnormalities* - Vitamin E deficiency can cause **neurological dysfunction** due to oxidative damage to neuronal membranes. - This often manifests as degeneration of the **posterior columns** of the spinal cord, leading to impaired proprioception and vibratory sensation. *Cerebellar ataxia* - The **cerebellum** is highly susceptible to oxidative stress, and vitamin E deficiency can lead to damage in this area. - This damage results in **ataxia**, characterized by impaired coordination, balance, and gait disturbances.

Question 772: Dietary deficiency of which vitamin is considered extremely rare?

A. Thiamine
B. Vitamin B6
C. Vitamin E (Correct Answer)
D. Vitamin D

Explanation: ***Vitamin E*** - **Vitamin E deficiency** is exceptionally rare because it is a **fat-soluble vitamin** stored in the body and is widely available in many common foods. - Symptoms of deficiency, when they do occur, are usually seen in individuals with severe **malabsorption syndromes** or genetic abnormalities affecting its metabolism. *Vitamin B6* - **Vitamin B6 deficiency** can occur, especially in individuals with **alcoholism**, those taking certain medications, or with certain chronic diseases. - It can manifest with neurological symptoms, such as **neuropathy** and **seizures**, as well as dermatological issues. *Thiamine* - **Thiamine (Vitamin B1) deficiency** is a known problem in regions with poor nutrition and in chronic alcoholics. - It leads to conditions like **beriberi** (wet and dry) and **Wernicke-Korsakoff syndrome**, affecting the cardiovascular and nervous systems. *Vitamin D* - **Vitamin D deficiency** is common globally, particularly in populations with limited sun exposure or inadequate dietary intake. - It can cause **rickets** in children and **osteomalacia** in adults, impacting bone health.

Question 773: Which nutrient is lost maximally in polished rice?

A. Proteins
B. Thiamine (Correct Answer)
C. Ascorbic acid
D. Calcitriol

Explanation: ***Thiamine*** - **Polishing rice** removes the outer layers (bran and germ), which are rich in **thiamine (vitamin B1)**. - Significant loss of thiamine can lead to **beriberi**, characterized by neurological and cardiovascular symptoms. *Proteins* - While some protein is lost during milling, the primary nutritional loss in polished rice is not protein. - The germ and aleurone layer contain some protein, but the main carbohydrate-rich endosperm remains. *Ascorbic acid* - **Ascorbic acid (Vitamin C)** is not a significant nutrient in rice, so its loss during polishing is negligible. - Rice is not a primary dietary source of vitamin C. *Calcitriol* - **Calcitriol** is the active form of **vitamin D**, and rice does not naturally contain vitamin D. - Therefore, it cannot be lost during the polishing process.

Question 774: Prolyl hydroxylase requires which cofactor?

A. Vitamin C (Correct Answer)
B. Iron (Fe²⁺)
C. Molybdenum
D. Vitamin K1

Explanation: ***Vitamin C*** - **Prolyl hydroxylase** is an enzyme critical for the hydroxylation of proline residues during **collagen synthesis**. - **Vitamin C** (ascorbic acid) acts as an essential **cofactor**, reducing the ferric iron of the enzyme back to its ferrous state after each catalytic cycle, enabling continued activity. - The enzyme requires both **iron (Fe²⁺)** as a metal cofactor and **vitamin C** to maintain the iron in its reduced state. *Iron (Fe²⁺)* - While **iron** is indeed required by prolyl hydroxylase as a **metal cofactor**, the question asks for the cofactor, which specifically refers to **vitamin C**. - Iron functions as part of the enzyme's active site, but vitamin C is the reducing agent that keeps iron functional. - Vitamin C deficiency (scurvy) leads to defective collagen synthesis despite adequate iron. *Molybdenum* - **Molybdenum** is a cofactor for several human enzymes, including **xanthine oxidase** and **sulfite oxidase**. - However, it plays no direct role in the activity of prolyl hydroxylase. *Vitamin K1* - **Vitamin K1** is a crucial cofactor for **gamma-glutamyl carboxylase**, an enzyme involved in the carboxylation of glutamic acid residues in clotting factors. - It is not involved in the hydroxylation of proline by prolyl hydroxylase.

Question 775: In the context of energy metabolism, which coenzyme is niacin a precursor to?

A. Thiamine pyrophosphate (TPP)
B. NADP
C. NAD (Correct Answer)
D. Flavin adenine dinucleotide (FAD)

Explanation: ***NAD*** - Niacin (vitamin B3) is a direct precursor to **nicotinamide adenine dinucleotide (NAD/NAD+)**. - NAD is the crucial coenzyme in **energy metabolism**, primarily involved in **catabolic pathways** such as glycolysis, TCA cycle, and electron transport chain. - Functions as an **electron carrier** in redox reactions, accepting electrons during oxidation of fuel molecules. *Thiamine pyrophosphate (TPP)* - **Thiamine (vitamin B1)** is the precursor to TPP, not niacin. - TPP plays a vital role in **carbohydrate metabolism**, particularly in pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase complexes. *NADP* - While niacin is also a precursor to **NADP/NADPH**, this coenzyme is primarily used in **anabolic (biosynthetic) pathways**, not energy metabolism. - NADP functions in reductive biosynthesis (fatty acid synthesis, cholesterol synthesis) and **oxidative stress protection** via the pentose phosphate pathway. - The question specifically asks about **energy metabolism**, making NAD the correct answer as it participates in catabolic, energy-producing reactions. *Flavin adenine dinucleotide (FAD)* - **Riboflavin (vitamin B2)** is the precursor to FAD, not niacin. - FAD is a coenzyme involved in various metabolic reactions, especially in the **TCA cycle** and **electron transport chain**, acting as an electron acceptor.

Question 776: Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?

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

Explanation: ***Vitamin D*** - The skin synthesizes vitamin D (specifically **cholecalciferol**) upon exposure to **ultraviolet B (UVB) radiation** from sunlight. - This endogenous production can be sufficient to meet physiological needs under adequate sun exposure, making it conditionally non-essential in the diet. *Vitamin K* - While **intestinal bacteria synthesize some vitamin K (K2)**, it is generally not in sufficient quantities to meet all physiological needs, especially for blood clotting. - Dietary intake of **vitamin K1 (phylloquinone)** from leafy green vegetables is critical. *Vitamin A* - **Vitamin A (retinol)** is obtained primarily from the diet, either directly from animal sources or from carotenoid precursors (like **beta-carotene**) in plants. - The body cannot synthesize vitamin A de novo; it relies on dietary intake and conversion from precursors. *Biotin* - Although the **gut microbiota can synthesize biotin**, the amount produced is generally considered insufficient to meet the body's requirements. - Therefore, biotin is primarily obtained through dietary intake, functioning as a coenzyme in various metabolic reactions.

Question 777: Which fat-soluble vitamin is most classically known for its steroid hormone-like action through nuclear receptors?

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

Explanation: ***Correct Answer: Vitamin D*** - **Vitamin D** (specifically its active form, **calcitriol** or **1,25-dihydroxyvitamin D₃**) is the **most classically recognized** fat-soluble vitamin that functions as a **steroid hormone** - It binds to the **vitamin D receptor (VDR)**, which is a member of the **nuclear receptor superfamily** - This VDR-calcitriol complex acts as a transcription factor, regulating gene expression involved in **calcium and phosphate homeostasis**, bone metabolism, skeletal development, and immune function - The mechanism is analogous to classic steroid hormones like cortisol, estrogen, and testosterone *Incorrect: Vitamin A* - **Vitamin A** (as **retinoic acid**) also interacts with nuclear receptors (**retinoic acid receptors - RARs** and **retinoid X receptors - RXRs**) to regulate gene transcription - However, Vitamin A is **most classically associated** with vision (rhodopsin in retinal photoreceptors), epithelial cell differentiation, embryonic development, and immune function - While it does have nuclear receptor-mediated actions, **Vitamin D is more prominently described** as having steroid hormone-like activity in standard medical education *Incorrect: Vitamin K* - **Vitamin K** functions primarily as a **cofactor for γ-glutamyl carboxylase**, an enzyme that catalyzes post-translational modification of glutamate residues to γ-carboxyglutamate (Gla) - Essential for the synthesis of **clotting factors** (II, VII, IX, X, protein C, protein S) and bone proteins (osteocalcin) - Does **not** act through nuclear receptors or function as a steroid hormone *Incorrect: Vitamin E* - **Vitamin E** (α-tocopherol) is a powerful **lipid-soluble antioxidant** that protects cell membranes from oxidative damage by scavenging free radicals - Functions primarily through its **antioxidant properties**, not through nuclear receptor binding - Does **not** have steroid hormone-like actions

Question 778: Which of the following elements have antioxidant properties?

A. Copper
B. Zinc
C. Selenium
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - **Selenium**, **copper**, and **zinc** all possess antioxidant properties, directly or indirectly, by being cofactors for various antioxidant enzymes or by directly scavenging free radicals. - These elements play crucial roles in maintaining **cellular redox balance** and protecting against **oxidative stress**. *Selenium* - It is a vital component of **glutathione peroxidase**, a key enzyme in the body's antioxidant defense system, which converts harmful **hydrogen peroxide** into water. - Selenium also contributes to the function of **thioredoxin reductases**, enzymes involved in regulating **redox signaling**. *Copper* - Copper is an essential cofactor for **superoxide dismutase (SOD1 and SOD3)**, an enzyme that catalyzes the dismutation of **superoxide radicals** into oxygen and hydrogen peroxide. - However, excessive copper can also act as a **pro-oxidant**, highlighting the importance of proper balance. *Zinc* - Zinc is another crucial cofactor for **superoxide dismutase (SOD1)** and is involved in protecting cells from **oxidative damage**. - It also stabilizes cell membranes, making them less susceptible to **oxidative damage**, and plays a role in regulating the expression of genes involved in **antioxidant defense**.

Question 779: Which of the following conditions is primarily treated with Vitamin B6?

A. Cystathionuria
B. None of the options
C. Xanthourenic aciduria
D. Homocystinuria (Correct Answer)

Explanation: ***Homocystinuria*** - The most common form of **homocystinuria** is caused by **cystathionine β-synthase (CBS) deficiency**, which requires **pyridoxal phosphate (Vitamin B6)** as a cofactor. - Approximately **50% of patients** with CBS deficiency are **B6-responsive**, making **high-dose Vitamin B6 (100-500 mg/day)** a **first-line primary treatment** for these cases. - This is a **clinically significant condition** that requires treatment, manifesting with features like **ectopia lentis, marfanoid habitus, intellectual disability, and thromboembolism**. - B6 supplementation enhances residual CBS enzyme activity and reduces plasma homocysteine levels. *Cystathionuria* - Caused by **cystathionine γ-lyase deficiency**, which also uses **Vitamin B6** as a cofactor. - This is generally a **benign, asymptomatic condition** that does **NOT require treatment**. - While B6 can reduce cystathionine accumulation, it is **not a primary treatment indication** because the condition is clinically insignificant. *Xanthurenic aciduria* - Caused by **kynureninase deficiency** in the **tryptophan metabolism pathway**, which requires **pyridoxal phosphate**. - This is a **rare and usually benign condition** that does not typically require treatment. - Not a primary indication for B6 therapy. *None of the options* - Incorrect, as **Homocystinuria** (CBS deficiency) is a **primary indication** for high-dose Vitamin B6 therapy in B6-responsive patients.

Question 780: What is a consequence of Vitamin E deficiency?

A. Ataxia (Correct Answer)
B. Cardiac failure
C. Hemorrhagic stroke
D. Megaloblastic anemia

Explanation: ***Ataxia*** - Vitamin E deficiency primarily affects the **nervous system** due to its role as an **antioxidant**, protecting cell membranes from oxidative damage. - Symptoms include **neurological deficits** such as **ataxia** (impaired coordination), **peripheral neuropathy**, and **muscle weakness**. *Cardiac failure* - While **severe nutritional deficiencies** can impact cardiac function, **vitamin E deficiency** is not a primary or common direct cause of **cardiac failure**. - **Thiamine deficiency** (beriberi) is a more classic cause of high-output cardiac failure. *Hemorrhagic stroke* - **Hemorrhagic stroke** is typically associated with conditions like **hypertension**, **aneurysms**, or **anticoagulant use**. - **Vitamin E** does not have a significant role in **blood clotting pathways** that would directly predispose to hemorrhage. *Megaloblastic anemia* - **Megaloblastic anemia** is characterized by abnormally large red blood cells and is primarily caused by deficiencies in **vitamin B12** or **folate**. - **Vitamin E deficiency** does not lead to this specific type of anemia.

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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