Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 681: Subacute combined degeneration of the spinal cord is a feature of which vitamin deficiency?

A. Vitamin A
B. Vitamin B6
C. Vitamin B9
D. Vitamin B12 (Correct Answer)

Explanation: **Explanation:** **Subacute Combined Degeneration (SCD)** of the spinal cord is a classic neurological manifestation of **Vitamin B12 (Cobalamin) deficiency**. **Why Vitamin B12 is the correct answer:** Vitamin B12 is a cofactor for the enzyme **Methylmalonyl-CoA mutase**, which converts methylmalonyl-CoA to succinyl-CoA. In B12 deficiency, methylmalonic acid (MMA) accumulates. This leads to the synthesis of abnormal fatty acids that are incorporated into neuronal lipids, causing **demyelination**. SCD specifically affects the **Posterior columns** (loss of vibration and position sense) and the **Lateral corticospinal tracts** (spastic paresis), hence the term "combined" degeneration. **Why the other options are incorrect:** * **Vitamin A:** Deficiency primarily leads to ocular symptoms like night blindness (nyctalopia) and xerophthalmia, not spinal cord degeneration. * **Vitamin B6 (Pyridoxine):** Deficiency typically causes peripheral neuropathy, sideroblastic anemia, and seizures, but does not cause SCD. * **Vitamin B9 (Folate):** While folate deficiency causes megaloblastic anemia identical to B12 deficiency, it **does not** cause neurological symptoms. Treating B12 deficiency with folate alone can worsen the neurological damage. **High-Yield Clinical Pearls for NEET-PG:** * **Schilling Test:** Historically used to diagnose the cause of B12 malabsorption. * **Pernicious Anemia:** The most common cause of B12 deficiency due to lack of Intrinsic Factor. * **Biochemical Markers:** In B12 deficiency, both **Homocysteine** and **Methylmalonic acid (MMA)** levels are elevated. In Folate deficiency, only Homocysteine is elevated. * **Peripheral Smear:** Look for hypersegmented neutrophils and macro-ovalocytes.

Question 682: Nyctalopia is due to deficiency of?

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

Explanation: **Explanation:** **Correct Answer: A. Vitamin A** Nyctalopia (night blindness) is the earliest clinical manifestation of Vitamin A deficiency. Vitamin A (Retinol) is essential for the synthesis of **Rhodopsin** (visual purple), a light-sensitive pigment found in the rod cells of the retina. Rods are responsible for vision in dim light. When light strikes the retina, Rhodopsin dissociates into opsin and all-trans-retinal (the Wald’s Visual Cycle). A deficiency in Vitamin A leads to an inability to regenerate Rhodopsin, resulting in impaired dark adaptation and night blindness. **Why other options are incorrect:** * **Vitamin B:** Deficiency typically leads to Beriberi (B1), Cheilosis/Glossitis (B2), Pellagra (B3), or Megaloblastic anemia (B12), but does not directly cause night blindness. * **Vitamin C:** Deficiency leads to **Scurvy**, characterized by defective collagen synthesis, bleeding gums, and impaired wound healing. * **Vitamin E:** Acts primarily as an antioxidant. Deficiency leads to hemolytic anemia and neurological symptoms (posterior column signs), but not nyctalopia. **NEET-PG High-Yield Pearls:** * **Sequence of Ocular Signs (WHO):** X1A (Conjunctival xerosis) → X1B (**Bitot’s spots**) → X2 (Corneal xerosis) → X3A/X3B (**Keratomalacia** - liquefaction of the cornea). * **Bitot’s Spots:** Triangular, foamy, pearly-white patches on the bulbar conjunctiva due to squamous metaplasia. * **Golden Rice:** A genetically modified variety of rice rich in Beta-carotene, developed to combat Vitamin A deficiency. * **Storage:** Vitamin A is stored in the **Ito cells** (Stellate cells) of the liver.

Question 683: Which of the following vitamins enhances intestinal absorption of calcium?

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

Explanation: **Explanation:** **Vitamin D (Cholecalciferol)** is the correct answer because it plays a pivotal role in calcium homeostasis. The active form of Vitamin D, **1,25-dihydroxycholicalciferol (Calcitriol)**, acts as a steroid hormone. It enters intestinal mucosal cells and binds to a nuclear receptor (VDR), promoting the transcription of **Calbindin-D9k**. This calcium-binding protein facilitates the transport of calcium across the enterocyte, significantly enhancing intestinal absorption of both calcium and phosphorus. **Why the other options are incorrect:** * **Vitamin K:** This is a fat-soluble vitamin essential for the post-translational gamma-carboxylation of clotting factors (II, VII, IX, X) and bone proteins like osteocalcin. It does not directly mediate intestinal calcium absorption. * **Vitamin B1 (Thiamine):** This is a water-soluble vitamin that acts as a coenzyme (TPP) for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase). Deficiency leads to Beriberi or Wernicke-Korsakoff syndrome. * **Vitamin B2 (Riboflavin):** This is a precursor for FMN and FAD, which are essential for redox reactions in the electron transport chain and TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The conversion of 25-OH-D3 to 1,25-(OH)2-D3 by the enzyme **1-alpha-hydroxylase** in the kidney is the most tightly regulated step in Vitamin D synthesis. * **Synergy:** Vitamin D works alongside Parathyroid Hormone (PTH) to maintain serum calcium levels. * **Deficiency:** Leads to **Rickets** in children (delayed osteoid mineralization) and **Osteomalacia** in adults (remodeling defect). * **Toxicity:** Excessive Vitamin D can lead to hypercalcemia and ectopic calcification (e.g., kidney stones).

Question 684: Which vitamin is supplied exclusively from animal sources?

A. Vitamin C
B. Vitamin B7
C. Vitamin B12 (Correct Answer)
D. Vitamin D

Explanation: **Explanation:** **Vitamin B12 (Cobalamin)** is unique among vitamins because it is synthesized exclusively by microorganisms (bacteria and archaea). In the food chain, it accumulates in animal tissues; therefore, it is found **exclusively in animal-derived foods** such as meat, eggs, dairy, and fish. There are no natural plant sources of B12, making strict vegans highly susceptible to deficiency unless they consume fortified foods or supplements. **Analysis of Incorrect Options:** * **Vitamin C (Ascorbic acid):** Primarily found in citrus fruits and green leafy vegetables. * **Vitamin B7 (Biotin):** Widely distributed in both plant (peanuts, soybeans) and animal (egg yolk) sources. It is also synthesized by intestinal flora. * **Vitamin D (Cholecalciferol):** While found in fatty fish and egg yolks, it is primarily synthesized endogenously in the skin via UV light exposure. **NEET-PG High-Yield Pearls:** * **Absorption:** Requires **Intrinsic Factor (IF)** secreted by gastric parietal cells. Absorption occurs in the **terminal ileum**. * **Storage:** Unlike other water-soluble vitamins, B12 is stored in the **liver** for 3–5 years. * **Clinical Correlation:** Deficiency leads to **Megaloblastic Anemia** and neurological symptoms (Subacute Combined Degeneration of the Spinal Cord) due to impaired myelin synthesis. * **Biochemical Role:** It is a coenzyme for two critical reactions: 1. Methionine synthase (Homocysteine → Methionine) 2. Methylmalonyl-CoA mutase (Methylmalonyl-CoA → Succinyl-CoA).

Question 685: Which vitamin deficiency can lead to lactic acidosis?

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

Explanation: ***Thiamine*** - **Thiamine (Vitamin B1)** is a crucial cofactor for the enzyme **pyruvate dehydrogenase (PDH)**, which converts **pyruvate** to **acetyl-CoA** to enter the Krebs cycle. - When thiamine is deficient, pyruvate cannot be processed efficiently, leading to its accumulation and subsequent shunting into **lactate** via **lactate dehydrogenase**, thus causing **lactic acidosis**, characteristic of **beriberi**. *Riboflavin* - Riboflavin (Vitamin B2) is a precursor for the coenzymes **FAD** and **FMN**, essential for redox reactions, but its deficiency symptoms mainly involve the oral cavity and skin. - Deficiency leads to **angular cheilitis**, **glossitis**, and **corneal vascularization**, and is not the direct cause of severe lactic acidosis. *Niacin* - Niacin (Vitamin B3) is a precursor for **NAD+** and **NADP+**, vital for many metabolic reactions. - Deficiency causes **pellagra**, characterized by the 3 D's: **Dermatitis**, **Diarrhea**, and **Dementia**, not primarily lactic acidosis. *Biotin* - Biotin (Vitamin B7) acts as a coenzyme for **carboxylase enzymes** (e.g., pyruvate carboxylase, acetyl-CoA carboxylase), which are necessary for gluconeogenesis and fatty acid synthesis. - While **pyruvate carboxylase** uses biotin, thiamine deficiency specifically impairs the key step of converting pyruvate to acetyl-CoA (via PDH complex), making it the primary cause of vitamin-deficiency-related lactic acidosis.

Question 686: Which of the following enzymes is dependent on Vitamin C for its activity?

A. Lysyl oxidase
B. Prolyl hydroxylase (Correct Answer)
C. Lysyl dehydrogenase
D. Hydroxyprolyl kinase

Explanation: ***Prolyl hydroxylase***- **Prolyl hydroxylase** requires **Vitamin C** (ascorbic acid) to reduce the ferric iron (Fe3+) in its active site back to **ferrous iron (Fe2+),** thereby maintaining its catalytic function.- This enzyme hydroxylates **proline** residues, a critical post-translational modification necessary for the stabilization and proper triple-helical winding of **collagen**; deficiency results in **scurvy**. *Lysyl dehydrogenase*- This enzyme is involved in **lysine catabolism** via deamination and is generally not associated with the direct role of Vitamin C in collagen synthesis.- It is distinct from **lysyl hydroxylase**, which *is* a Vitamin C-dependent enzyme modifying lysine in collagen. *Lysyl oxidase*- **Lysyl oxidase** is responsible for forming covalent **cross-links** between collagen and elastin molecules, stabilizing the extracellular matrix structure.- This enzyme requires **copper** (Cu2+) as a cofactor for its activity and is not directly dependent on Vitamin C. *Hydroxyprolyl kinase*- This is not a standard physiological enzyme involved in the key metabolic pathways of **collagen synthesis** or degradation.- Kinase activity concerns **phosphorylation**, which is unrelated to the **hydroxylation** mechanism essential for Vitamin C dependence in connective tissue metabolism.

Question 687: A patient presents with gum bleeding, petechiae, poor wound healing, and other related symptoms. Which of the following vitamin deficiencies is most likely responsible for this presentation?

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

Explanation: ***Vitamin C*** - Vitamin C (ascorbic acid) is essential for the hydroxylation of **proline** and **lysine** residues during collagen synthesis, crucial for maintaining the integrity of blood vessels and skin. - Deficiency results in **scurvy**, characterized by defective collagen, leading to fragile capillaries (manifesting as **gum bleeding** and **petechiae**) and impaired **wound healing**. *Vitamin A* - Deficiency is primarily associated with visual disturbances, most notably **night blindness** (**nyctalopia**), and **xerophthalmia** (dry eyes). - Its main roles involve vision, epithelial differentiation, and immune function, not the stability of subendothelial collagen required to prevent bleeding tendencies described here. *Vitamin B6* - Pyridoxine deficiency symptoms include **seborrheic dermatitis**, **stomatitis**, **cheilosis**, and neurological issues like **peripheral neuropathy** and seizures in infants. - While involved in numerous metabolic pathways, its deficiency is not responsible for the characteristic symptoms of capillary fragility (petechiae and bleeding). *Vitamin B3* - Niacin deficiency causes **pellagra**, classically identified by the "3 D's": **Dermatitis** (photosensitive rash), **Diarrhea**, and **Dementia**. - Although pellagra involves epithelial changes (dermatitis), it does not typically present with the severe vascular fragility, gum manifestations, and poor generalized wound healing seen in scurvy.

Question 688: A 34-year-old male whose staple diet primarily consisted of only maize presents with diarrhea, dementia, and photosensitive dermatitis in sun-exposed areas. Which vitamin is most likely deficient in this patient?

A. Niacin (Correct Answer)
B. Vitamin B6
C. Ascorbic acid
D. Biotin

Explanation: **Correct: Niacin** - The constellation of **dementia**, **diarrhea**, and **photosensitive dermatitis** (the 3 Ds) is the hallmark presentation of **Pellagra**. - A diet reliant on **maize** (corn) is a classic cause of Pellagra because maize is deficient in **tryptophan**, a vital precursor to Niacin (Vitamin B3). - Pellagra can progress to a fourth D (**Death**) if untreated. *Incorrect: Vitamin B6* - Deficiency typically manifests as **peripheral neuropathy**, **seborrheic dermatitis** (non-photosensitive), and microcytic anemia (**sideroblastic anemia**). - It is not associated with the severe chronic diarrhea or the characteristic sun-exposed rash seen in Pellagra. *Incorrect: Ascorbic acid* - Deficiency causes **Scurvy**, characterized by **bleeding gums**, **perifollicular hemorrhages** (petechiae), and **impaired wound healing**. - Neurological symptoms typical of dementia and pronounced gastrointestinal symptoms are not primary features of Scurvy. *Incorrect: Biotin* - Biotin (Vitamin B7) deficiency is rare, usually presenting with **alopecia**, non-specific dermatitis, and sometimes hypotonia or developmental delay. - It is differentiated by the absence of characteristic photosensitive rash and the severe central nervous system symptoms (dementia) that define this case.

Question 689: Which of the following vitamins functions as a coenzyme in transamination reactions?

A. Vitamin B1 (Thiamine)
B. Vitamin B6 (Pyridoxine) (Correct Answer)
C. Vitamin B12 (Cobalamin)
D. Vitamin B2 (Riboflavin)

Explanation: ***Vitamin B6 (Pyridoxine)*** - **Pyridoxal phosphate (PLP)**, the active coenzyme form of Vitamin B6, is absolutely essential for **transamination reactions** (transfer of amino groups from an amino acid to a keto acid), making it critical in amino acid metabolism. - PLP is also involved in other enzymatic reactions, including decarboxylation and racemization of amino acids, and is crucial for **heme synthesis** and **neurotransmitter synthesis**. *Vitamin B1 (Thiamine)* - Functions primarily as **thiamine pyrophosphate (TPP)**, which is a coenzyme for enzymes involved in the transfer of **aldehyde groups**, such as **pyruvate dehydrogenase** and alpha-ketoglutarate dehydrogenase (oxidative decarboxylation). - Deficiency leads to Beriberi (wet, dry, and infantile) and **Wernicke-Korsakoff syndrome**. *Vitamin B12 (Cobalamin)* - Serves as a coenzyme in **methylation reactions** (methionine synthase) and **isomerization reactions** (methylmalonyl-CoA mutase). - Essential for **DNA synthesis**, **red blood cell formation**, and **neurological function**. - Deficiency leads to **megaloblastic anemia** and **subacute combined degeneration** of the spinal cord. *Vitamin B2 (Riboflavin)* - Serves as a precursor for the coenzymes **flavin mononucleotide (FMN)** and **flavin adenine dinucleotide (FAD)**. - These coenzymes are crucial for various **oxidation-reduction reactions** (electron transfer) in intermediary metabolism, such as those catalyzed by succinate dehydrogenase.

Question 690: A patient presents with anemia, positive Romberg sign, and other neurological findings suggestive of vitamin B12 deficiency. Laboratory findings show elevated homocysteine levels. Which amino acid is likely to be deficient in this patient?

A. Methionine (Correct Answer)
B. Tyrosine
C. Cysteine
D. Glutamate

Explanation: ***Methionine*** - Vitamin B12 is an essential cofactor for the enzyme **methionine synthase**, which converts **homocysteine** into methionine. - A deficiency in B12 blocks this essential metabolic step, leading to the accumulation of homocysteine and a resultant deficiency of **methionine**. *Cysteine* - **Cysteine** is derived from methionine via **homocysteine** and requires **Vitamin B6** (pyridoxal phosphate) for its synthesis, not Vitamin B12 directly. - While B12 deficiency results in increased homocysteine which is an intermediate, the direct deficiency relates to the product of the B12-dependent step, **methionine**. *Tyrosine* - **Tyrosine** is synthesized from **phenylalanine** by the enzyme phenylalanine hydroxylase, a pathway entirely separate from **Vitamin B12** and homocysteine metabolism. - It is generally considered a non-essential amino acid whose metabolism is disturbed primarily in conditions like **Phenylketonuria (PKU)**. *Glutamate* - **Glutamate** is a crucial excitatory neurotransmitter whose synthesis and metabolism are not directly linked to the **methionine synthase** reaction or **Vitamin B12** deficiency. - Neurological symptoms associated with B12 deficiency (like Romberg sign) are due to impaired **myelin synthesis** caused by reduced SAM (derived from methionine) and methylmalonyl-CoA accumulation.

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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