Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 741: A 55-year-old woman presents with frequent infections and delayed wound healing. Laboratory tests reveal neutropenia and low levels of ascorbic acid. What is the most likely cause of her symptoms?

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

Explanation: ***Vitamin C deficiency*** - **Vitamin C** (ascorbic acid) is crucial for **collagen synthesis**, essential for wound healing, which explains the delayed wound healing in this patient. - It plays a vital role in **immune function**, particularly in **neutrophil function** (chemotaxis and microbicidal activity), which can contribute to frequent infections. - Low levels of ascorbic acid directly confirm this deficiency, which can lead to **scurvy** in severe cases, characterized by impaired immune response and connective tissue weakness. - While neutropenia is not a classic feature of vitamin C deficiency, severe deficiency can impair neutrophil function and overall immune competence. *Vitamin A deficiency* - Primarily affects **vision** (e.g., night blindness) and **epithelial integrity**, leading to xerophthalmia and keratinization of epithelial surfaces. - While it can impair immune function, it doesn't typically present with neutropenia or the specific laboratory finding of low ascorbic acid levels. *Vitamin D deficiency* - Primarily affects **bone metabolism** and **calcium homeostasis**, leading to conditions like **osteomalacia** or **rickets**. - While it plays a role in immune modulation, it does not typically cause neutropenia or significantly delayed wound healing, and would not cause low ascorbic acid levels. *Vitamin K deficiency* - Primarily affects **blood coagulation** due to its role in synthesizing clotting factors (II, VII, IX, X). - Symptoms usually include **bleeding tendencies** (e.g., easy bruising, prolonged bleeding), not frequent infections, neutropenia, or delayed wound healing.

Question 742: What is the impact of vitamin K deficiency on blood clotting factors, and which specific factors are primarily affected?

A. Inhibits γ-carboxylation of clotting factors II, VII, IX, and X (Correct Answer)
B. Reduces synthesis of clotting factors VIII and IX
C. Increases degradation of clotting factors V and VIII
D. Impedes conversion of prothrombin to thrombin

Explanation: ***Inhibits γ-carboxylation of clotting factors II, VII, IX, and X*** - Vitamin K is a crucial cofactor for the **γ-carboxylation** of specific glutamic acid residues on clotting factors **II (prothrombin)**, **VII**, **IX**, and **X**. - This post-translational modification is essential for these factors to bind calcium and phospholipids, which is necessary for their activation and function in the coagulation cascade. *Reduces synthesis of clotting factors VIII and IX* - Vitamin K deficiency primarily affects the **post-translational modification** (γ-carboxylation) rather than the synthesis rate of clotting factors. - While factor IX is vitamin K-dependent, its synthesis is not directly reduced, but its function is impaired due to a lack of proper carboxylation. Factor VIII is not vitamin K-dependent. *Increases degradation of clotting factors V and VIII* - Vitamin K deficiency does not directly increase the degradation of clotting factors **V** and **VIII**. - These factors are not vitamin K-dependent and their levels are primarily regulated by other mechanisms in the coagulation cascade. *Impedes conversion of prothrombin to thrombin* - This statement is partially correct but not the primary or most direct impact. The impeded conversion of prothrombin to thrombin is a *consequence* of the functional deficiency of vitamin K-dependent clotting factors, especially **factor II (prothrombin)** itself. - The direct impact of vitamin K deficiency is the impaired **γ-carboxylation** of prothrombin, which then prevents its proper activation and subsequent conversion into thrombin.

Question 743: Which vitamin acts as a coenzyme for the enzyme pyruvate dehydrogenase?

A. Vitamin B1 (Correct Answer)
B. Vitamin B2
C. Vitamin B3
D. Vitamin B5

Explanation: ***Vitamin B1*** - **Thiamine (Vitamin B1)** is the precursor to **thiamine pyrophosphate (TPP)**, which acts as the coenzyme for **pyruvate dehydrogenase** (the E1 component). - TPP is essential for the oxidative decarboxylation of pyruvate to form acetyl-CoA, linking glycolysis to the citric acid cycle. - This is the **primary and direct coenzyme** for the pyruvate dehydrogenase enzyme itself. *Vitamin B2* - **Riboflavin (Vitamin B2)** forms **FAD**, which serves as a cofactor for the **E3 component** (dihydrolipoyl dehydrogenase) of the pyruvate dehydrogenase complex. - While FAD is essential for the overall complex function, it is not the coenzyme for the pyruvate dehydrogenase enzyme (E1) itself. *Vitamin B3* - **Niacin (Vitamin B3)** forms **NAD+**, which is also a cofactor for the **E3 component** of the PDH complex. - NAD+ accepts electrons during the reoxidation reactions within the complex but does not directly act on the E1 enzyme. *Vitamin B5* - **Pantothenic acid (Vitamin B5)** is a component of **Coenzyme A (CoA)**, which serves as a substrate (not a coenzyme) for the complex. - CoA accepts the acetyl group to form acetyl-CoA, the final product, but does not catalyze the dehydrogenase reaction itself.

Question 744: Which vitamin is essential for the synthesis of neurotransmitters such as serotonin and norepinephrine, and is involved in amino acid metabolism?

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

Explanation: **Vitamin B6** - **Pyridoxal phosphate (PLP)**, the active form of vitamin B6, is a crucial coenzyme in the synthesis of several neurotransmitters, including **serotonin**, **norepinephrine**, **dopamine**, and GABA. - It plays a vital role in **amino acid metabolism**, including transamination, decarboxylation, and racemization reactions, which are essential for neurological function. *Vitamin B1* - Also known as **thiamine**, vitamin B1 is primarily involved in **carbohydrate metabolism** and energy production (e.g., in the pyruvate dehydrogenase complex). - While important for nerve function, its direct role in neurotransmitter synthesis is not as prominent as that of vitamin B6. *Vitamin B12* - **Cobalamin** (vitamin B12) is essential for DNA synthesis, red blood cell formation, and the maintenance of the **myelin sheath** around nerves. - Its involvement in neurotransmitter synthesis is indirect, mainly through its role in the metabolism of **S-adenosylmethionine (SAM)**, a methyl donor for various reactions. *Vitamin C* - **Ascorbic acid** (vitamin C) is a powerful antioxidant and is critical for **collagen synthesis** and immune function. - It acts as a co-factor for some enzymes involved in neurotransmitter synthesis, particularly the conversion of dopamine to norepinephrine, but its primary metabolic roles are distinct from those of vitamin B6.

Question 745: What is the primary biochemical role of riboflavin (vitamin B2) in cellular metabolism?

A. It serves as a precursor for the synthesis of steroid hormones.
B. It is involved in the transport of oxygen in red blood cells.
C. It functions as a coenzyme in oxidation-reduction reactions. (Correct Answer)
D. It regulates the transcription of DNA.

Explanation: **Correct: It functions as a coenzyme in oxidation-reduction reactions.** - Riboflavin is a crucial precursor for the coenzymes **flavin adenine dinucleotide (FAD)** and **flavin mononucleotide (FMN)**. - These coenzymes are essential in many **redox reactions** in metabolic pathways, including the **electron transport chain**, playing a vital role in **cellular energy production**. - FAD/FMN act as electron carriers, accepting and donating electrons in oxidation-reduction reactions throughout metabolism. *Incorrect: It serves as a precursor for the synthesis of steroid hormones.* - **Cholesterol** is the primary precursor for steroid hormones (cortisol, aldosterone, sex hormones), not riboflavin. - Riboflavin's biochemical role is distinct from hormone synthesis pathways. *Incorrect: It is involved in the transport of oxygen in red blood cells.* - **Hemoglobin**, which contains iron and heme groups, is responsible for oxygen transport in red blood cells. - Riboflavin does not directly participate in oxygen binding or transport mechanisms. *Incorrect: It regulates the transcription of DNA.* - **Transcription factors** and **RNA polymerase** are the primary regulators of DNA transcription. - While some vitamins influence gene expression indirectly, riboflavin's primary and direct role is as a redox coenzyme, not in transcriptional regulation.

Question 746: A deficiency in which vitamin is most likely to impair the detoxification of free radicals?

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

Explanation: ***Vitamin E*** - **Vitamin E** is a primary **lipid-soluble antioxidant** that plays a crucial role in protecting cell membranes from **oxidative damage** by scavenging free radicals. - A deficiency would therefore directly impair the body's ability to **detoxify reactive oxygen species**. *Vitamin A* - **Vitamin A** is important for vision, immune function, and cell growth, and while it has some antioxidant properties, its primary role is not free radical detoxification. - Its main roles involve **retinal function** and maintaining **epithelial tissues**. *Vitamin D* - **Vitamin D** is crucial for bone health and calcium homeostasis, and also plays a role in immune regulation. - It does not significantly contribute to the direct **detoxification of free radicals**. *Vitamin K* - **Vitamin K** is vital for blood clotting and bone metabolism. - It has no known direct role in the **scavenging of free radicals**.

Question 747: Which vitamin deficiency is associated with the development of pellagra?

A. Vitamin B1
B. Vitamin B2
C. Vitamin B3 (Correct Answer)
D. Vitamin B6

Explanation: ***Vitamin B3*** - Pellagra is classically associated with a deficiency of **niacin (Vitamin B3)**, often presenting with the "4 Ds": **dermatitis, diarrhea, dementia, and death**. - Niacin is crucial for **nicotinamide adenine dinucleotide (NAD)** and **nicotinamide adenine dinucleotide phosphate (NADP)** synthesis, essential coenzymes in metabolic pathways. *Vitamin B1* - Deficiency of **thiamine (Vitamin B1)** leads to **beriberi**, characterized by cardiovascular (wet beriberi) or neurological (dry beriberi) symptoms, distinct from pellagra. - It is critical for **carbohydrate metabolism** and nerve function. *Vitamin B2* - Deficiency of **riboflavin (Vitamin B2)** results in **ariboflavinosis**, which can cause **cheilosis**, **angular stomatitis**, and **glossitis**, but not the classic widespread dermatitis and neurological symptoms of pellagra. - Riboflavin is a precursor for **flavin adenine dinucleotide (FAD)** and **flavin mononucleotide (FMN)**, important for redox reactions. *Vitamin B6* - Deficiency of **pyridoxine (Vitamin B6)** can cause **peripheral neuropathy**, **seborrheic dermatitis-like rash**, and **anemia**, but it does not typically manifest as pellagra. - Vitamin B6 is involved in **amino acid metabolism** and neurotransmitter synthesis.

Question 748: What is the primary biochemical consequence of a deficiency in vitamin B12?

A. Impaired DNA synthesis (Correct Answer)
B. Decreased fatty acid synthesis
C. Reduced protein translation
D. Enhanced glycolysis

Explanation: ***Impaired DNA synthesis*** - Vitamin B12 is essential for the conversion of **methylmalonyl-CoA** to **succinyl-CoA** and the methylation of **homocysteine to methionine**. - The latter reaction, catalyzed by **methionine synthase**, is crucial for regenerating **tetrahydrofolate**, which is necessary for **purine and pyrimidine synthesis** (components of DNA). *Decreased fatty acid synthesis* - While vitamin B12 is involved in **fatty acid metabolism** (specifically odd-chain fatty acids), its primary and most significant biochemical consequence in deficiency is not decreased overall fatty acid synthesis. - **Malonyl-CoA** and **acetyl-CoA carboxylase** are key enzymes in fatty acid synthesis, not directly dependent on B12 for their primary activity. *Reduced protein translation* - Protein translation primarily relies on **ribosomes**, **tRNA**, **mRNA**, and amino acids, with its regulation largely independent of vitamin B12's direct enzymatic roles. - While prolonged deficiency can indirectly affect overall cellular function and thus protein synthesis, it's not the immediate primary biochemical consequence. *Enhanced glycolysis* - Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating ATP and NADH. - Vitamin B12 is not a direct cofactor or regulator in the glycolytic pathway; therefore, its deficiency would not directly enhance this process.

Question 749: What is the coenzyme form of the vitamin niacin?

A. Thiamine pyrophosphate
B. Flavin adenine dinucleotide
C. Nicotinamide adenine dinucleotide (Correct Answer)
D. Pyridoxal phosphate

Explanation: ***Nicotinamide adenine dinucleotide*** - **Niacin** (vitamin B3) is converted to **nicotinamide adenine dinucleotide (NAD+)**, which is the **principal coenzyme form** of niacin. - NAD+ and its phosphorylated derivative **NADP+** are both coenzymes derived from niacin. - These coenzymes are crucial for numerous metabolic redox reactions, acting as **electron carriers** in processes like **glycolysis**, the **Krebs cycle**, **oxidative phosphorylation**, and biosynthetic pathways. - **NAD+/NADH** is primarily involved in **catabolic reactions** (energy production), while **NADP+/NADPH** is mainly used in **anabolic reactions** (biosynthesis). *Thiamine pyrophosphate* - This is the active coenzyme form of **thiamine (vitamin B1)**. - It plays a vital role in carbohydrate metabolism, particularly in the **decarboxylation of alpha-keto acids**, such as in the pyruvate dehydrogenase complex and alpha-ketoglutarate dehydrogenase complex. *Flavin adenine dinucleotide* - This is a coenzyme derived from **riboflavin (vitamin B2)**. - **FAD** and its reduced form **FADH2** are important electron carriers in various metabolic pathways, including the **electron transport chain** and beta-oxidation of fatty acids. *Pyridoxal phosphate* - This is the active coenzyme form of **pyridoxine (vitamin B6)**. - It is essential for a wide range of enzymatic reactions, especially those involving **amino acid metabolism**, such as **transamination**, **decarboxylation**, and **transsulfuration** reactions.

Question 750: Which of the following is NOT a role of vitamin D in bone metabolism?

A. Enhances collagen synthesis in osteoblasts (Correct Answer)
B. Promotes phosphate absorption in the intestines
C. Influences bone resorption indirectly through calcium regulation
D. Promotes calcium absorption in the intestine

Explanation: ***Enhances collagen synthesis in osteoblasts*** - While **vitamin D** is crucial for **bone health**, its primary role is not directly in enhancing **collagen synthesis** in osteoblasts. - **Collagen synthesis** is mainly influenced by factors like **vitamin C** and **growth hormones**, which are critical for the organic matrix of bone. *Promotes phosphate absorption in the intestines* - **Vitamin D** plays a vital role in maintaining **phosphate homeostasis** by increasing its absorption from the intestine. - This increased **phosphate**, along with **calcium**, is essential for **bone mineralization**. *Influences bone resorption indirectly through calcium regulation* - **Vitamin D** helps maintain **serum calcium levels**, which in turn influences the activity of **parathyroid hormone (PTH)**. - High **PTH** levels, often stimulated by low **calcium**, lead to increased **bone resorption** to release calcium into the blood. *Promotes calcium absorption in the intestine* - This is a well-established and critical function of **vitamin D**, facilitated by its active form, **calcitriol**. - Increased intestinal **calcium absorption** is essential for maintaining **serum calcium levels** and providing the necessary building blocks for **bone mineralization**.

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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