Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 461: Vitamin D deficiency has all of the following effects EXCEPT?

A. Hypocalcemia
B. Increased serum alkaline phosphatase
C. Increased parathyroid hormone
D. Hyperphosphatemia (Correct Answer)

Explanation: **Explanation:** Vitamin D plays a crucial role in maintaining calcium and phosphate homeostasis. The hallmark of Vitamin D deficiency is **Hypophosphatemia**, not hyperphosphatemia. **1. Why Hyperphosphatemia is the Correct Answer (The "Except"):** Vitamin D (Calcitriol) normally increases the intestinal absorption of both Calcium and Phosphorus. In deficiency, absorption of both minerals decreases. Furthermore, the resulting low serum calcium triggers **Secondary Hyperparathyroidism**. Parathyroid Hormone (PTH) acts on the kidneys to increase phosphate excretion (phosphaturia), leading to low serum phosphate levels (**Hypophosphatemia**). **2. Analysis of Incorrect Options:** * **A. Hypocalcemia:** Vitamin D is essential for intestinal calcium absorption via calbindin. Its deficiency directly leads to reduced serum calcium levels. * **B. Increased Serum Alkaline Phosphatase (ALP):** This is a sensitive marker for increased osteoblastic activity. In Vitamin D deficiency (Rickets/Osteomalacia), there is a failure of osteoid mineralization, leading to a compensatory increase in ALP. * **C. Increased Parathyroid Hormone:** As serum calcium drops, the parathyroid glands compensate by secreting more PTH (Secondary Hyperparathyroidism) to mobilize calcium from bones and increase renal reabsorption. **Clinical Pearls for NEET-PG:** * **Biochemical Profile of Vitamin D Deficiency:** ↓ Calcium, ↓ Phosphate, ↑ PTH, and ↑ ALP. * **Earliest Sign:** An increase in Serum ALP is often the earliest biochemical marker of Rickets before radiological changes appear. * **Key Enzyme:** 1-alpha-hydroxylase in the kidney converts 25(OH)D to the active form 1,25(OH)₂D (Calcitriol). This enzyme is stimulated by PTH. * **Rickets vs. Renal Failure:** In Chronic Kidney Disease (Renal Osteodystrophy), you see **Hyperphosphatemia** because the kidneys cannot excrete phosphate, which distinguishes it from nutritional Vitamin D deficiency.

Question 462: In pyridoxine deficiency, tryptophan is converted to?

A. Nicotine
B. Acetoacetate
C. Xantheurenic acid (Correct Answer)
D. Acetyl CoA

Explanation: ### Explanation **Concept:** The metabolism of Tryptophan follows the **Kynurenine pathway** to synthesize Niacin (Vitamin B3). A critical step in this pathway is the conversion of **3-hydroxykynurenine to 3-hydroxyanthranilic acid**, catalyzed by the enzyme **Kynureninase**. This enzyme is strictly dependent on **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6. In **Pyridoxine (B6) deficiency**, Kynureninase activity is impaired. As a result, 3-hydroxykynurenine cannot follow its normal path and is instead shunted toward an alternative metabolic route. It undergoes transamination to form **Xanthurenic acid**, which is then excreted in the urine. **Analysis of Options:** * **B. Acetoacetate & D. Acetyl CoA:** Tryptophan is both glucogenic and ketogenic. Under normal physiological conditions, it eventually breaks down into these intermediates. However, in B6 deficiency, the block occurs much earlier in the pathway, leading to the abnormal accumulation of xanthurenic acid rather than these end products. * **A. Nicotine:** This is a distractor. Tryptophan is a precursor for **Nicotinic acid (Niacin)**, not nicotine (an alkaloid found in tobacco). **Clinical Pearls for NEET-PG:** 1. **The Xanthurenic Acid Test:** Urinary excretion of xanthurenic acid after a Tryptophan load is a sensitive diagnostic marker for Vitamin B6 deficiency. 2. **Pellagra Connection:** Since B6 is required to convert Tryptophan to Niacin, a chronic B6 deficiency can manifest as **secondary Pellagra** (Dermatitis, Diarrhea, Dementia). 3. **Isoniazid (INH) Link:** Patients on INH for Tuberculosis often develop B6 deficiency (INH binds PLP), leading to peripheral neuropathy and increased xanthurenic acid excretion.

Question 463: Vitamin K is available as all the following except?

A. Menoquinone
B. Menadione
C. Phytonadione
D. Phytoquinone (Correct Answer)

Explanation: **Explanation:** Vitamin K exists in several natural and synthetic forms. The question asks for the "except" option, and **Phytoquinone** is the correct answer because it is a non-existent or misnamed term in medical biochemistry. The natural plant-derived form is actually called **Phylloquinone**. **Analysis of Options:** * **Phytonadione (Option C):** This is the pharmaceutical name for **Vitamin K1 (Phylloquinone)**. It is the natural form found in green leafy vegetables and is the primary dietary source. * **Menoquinone (Option A):** Also known as **Menaquinone (Vitamin K2)**, this form is synthesized by intestinal bacterial flora. It is essential for bone metabolism and cardiovascular health. * **Menadione (Option B):** This is **Vitamin K3**, a synthetic, water-soluble analogue. Unlike K1 and K2, it is provitamin-like and must be converted to an active form in the body. Note: It is rarely used clinically now due to the risk of hemolytic anemia and neonatal jaundice. **Clinical Pearls for NEET-PG:** 1. **Biochemical Function:** Vitamin K acts as a coenzyme for **$\gamma$-glutamyl carboxylase**, which converts glutamic acid residues to $\gamma$-carboxyglutamate (Gla) on clotting factors. 2. **Clotting Factors:** It is essential for the post-translational modification of **Factors II, VII, IX, X**, and **Proteins C and S**. 3. **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the regeneration of active (reduced) Vitamin K. 4. **Deficiency:** Leads to an increased Prothrombin Time (PT/INR). Newborns are routinely given a Vitamin K injection at birth to prevent **Hemorrhagic Disease of the Newborn** because the placenta transfers Vitamin K poorly and the neonatal gut is sterile.

Question 464: Korsakoff's syndrome is due to deficiency of?

A. Thiamine (Correct Answer)
B. Niacin
C. Folate
D. Pyridoxin

Explanation: **Explanation:** **Korsakoff’s syndrome** is a chronic neuropsychiatric manifestation of **Thiamine (Vitamin B1)** deficiency, most commonly seen in chronic alcoholics. Thiamine, in its active form **Thiamine Pyrophosphate (TPP)**, is a crucial coenzyme for key enzymes in carbohydrate metabolism: Pyruvate Dehydrogenase, $\alpha$-ketoglutarate dehydrogenase, and Transketolase. Deficiency leads to impaired glucose utilization in the brain, causing neuronal damage, particularly in the **mammillary bodies** and dorsomedial nucleus of the thalamus. Clinically, it is characterized by anterograde amnesia and **confabulation** (filling memory gaps with fabricated stories). **Why other options are incorrect:** * **Niacin (B3):** Deficiency leads to **Pellagra**, characterized by the 4 D's: Dermatitis, Diarrhea, Dementia, and Death. * **Folate (B9):** Deficiency causes **Megaloblastic anemia** and neural tube defects in fetuses, but not specific amnestic syndromes. * **Pyridoxine (B6):** Deficiency typically presents with peripheral neuropathy, sideroblastic anemia, and seizures (due to decreased GABA synthesis), but not Korsakoff’s. **High-Yield Clinical Pearls for NEET-PG:** * **Wernicke-Korsakoff Syndrome (WKS):** Wernicke’s Encephalopathy is the acute, reversible phase (Triad: Ataxia, Ophthalmoplegia, Confusion). Korsakoff’s is the chronic, often irreversible phase. * **The "Glucose Rule":** Always administer Thiamine *before* or *with* IV Glucose in malnourished patients. Giving glucose alone can precipitate Wernicke’s by consuming the remaining TPP stores. * **Diagnostic Marker:** Decreased **Erythrocyte Transketolase activity** is a sensitive indicator of Thiamine deficiency.

Question 465: Cysteine is considered a non-essential amino acid only in the presence of dietary?

A. Methionine (Correct Answer)
B. Serine
C. Folate
D. Phenylalanine

Explanation: **Explanation:** The synthesis of **Cysteine** is a classic example of the metabolic interplay between essential and non-essential amino acids. Cysteine is synthesized via the **transsulfuration pathway**, which requires two primary precursors: 1. **Methionine (Essential):** Provides the sulfur atom. 2. **Serine (Non-essential):** Provides the carbon skeleton. **Why Methionine is the correct answer:** Cysteine is termed "semi-essential" or "conditionally essential." It can only be synthesized in the body if there is an adequate supply of **Methionine**. Methionine is converted to S-adenosylmethionine (SAM) and then to Homocysteine. Homocysteine then combines with Serine to form Cystathionine (catalyzed by Cystathionine $\beta$-synthase), which is eventually cleaved to yield Cysteine. If dietary Methionine is deficient, the body cannot produce the sulfur required for Cysteine, making Cysteine an essential requirement in the diet. **Analysis of Incorrect Options:** * **B. Serine:** While Serine provides the carbon skeleton for Cysteine, it is a non-essential amino acid that the body can synthesize from glucose (via 3-phosphoglycerate). Therefore, its dietary presence is not mandatory for Cysteine synthesis. * **C. Folate:** Folate is involved in the *remethylation* of homocysteine back to methionine, not the direct synthesis of the cysteine carbon-sulfur bond. * **D. Phenylalanine:** This is an essential amino acid required for the synthesis of **Tyrosine**, not Cysteine. **High-Yield Clinical Pearls for NEET-PG:** * **Homocystinuria:** Most commonly due to a deficiency of **Cystathionine $\beta$-synthase**. In these patients, Cysteine becomes a **strictly essential** amino acid because the pathway to synthesize it is blocked. * **Cofactor Requirement:** The transsulfuration pathway (Cystathionine $\beta$-synthase) requires **Vitamin B6 (Pyridoxine)**. * **Sparing Action:** Dietary Cysteine has a "sparing effect" on Methionine, as it reduces the amount of Methionine that must be diverted toward Cysteine synthesis.

Question 466: Which vitamin is associated with glycogen phosphorylase?

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

Explanation: **Explanation:** **Pyridoxine (Vitamin B6)** is the correct answer because its active form, **Pyridoxal Phosphate (PLP)**, is an essential cofactor for **Glycogen Phosphorylase**, the rate-limiting enzyme of glycogenolysis. Unlike most PLP-dependent enzymes that involve amino acid metabolism, in glycogen phosphorylase, the phosphate group of PLP acts as a general acid-base catalyst to facilitate the phosphorolysis of glycogen into glucose-1-phosphate. Notably, more than 80% of the body’s total vitamin B6 is stored in skeletal muscle, bound to this specific enzyme. **Why other options are incorrect:** * **Niacin (B3):** Functions as NAD/NADP, primarily involved in redox reactions (e.g., glycolysis, TCA cycle). * **Riboflavin (B2):** Functions as FAD/FMN, acting as a prosthetic group for oxidoreductases (e.g., Succinate dehydrogenase). * **Thiamine (B1):** Functions as TPP, a cofactor for oxidative decarboxylation (e.g., Pyruvate dehydrogenase) and the transketolase reaction in the HMP shunt. **High-Yield Clinical Pearls for NEET-PG:** 1. **PLP & Amino Acids:** PLP is the universal cofactor for **Transamination** (ALT/AST), **Decarboxylation** (e.g., Histidine to Histamine), and **Deamination**. 2. **Heme Synthesis:** PLP is a cofactor for **ALA Synthase**, the rate-limiting step of heme synthesis. Deficiency leads to **Sideroblastic Anemia**. 3. **Drug Interaction:** **Isoniazid (INH)** therapy for TB induces B6 deficiency by forming inactive complexes, leading to peripheral neuropathy. Always co-prescribe B6 with INH. 4. **Neurotransmitters:** B6 is required for the synthesis of GABA, Serotonin, Epinephrine, and Dopamine.

Question 467: Casal necklace is seen in which condition?

A. Scurvy
B. Kwashiorkor
C. Pellagra (Correct Answer)
D. Indicanuria

Explanation: **Explanation:** **Casal’s necklace** is a pathognomonic clinical sign of **Pellagra**, which is caused by a deficiency of **Niacin (Vitamin B3)** or its precursor amino acid, **Tryptophan**. In Pellagra, the skin becomes hypersensitive to sunlight (photosensitivity). This leads to a characteristic symmetric, pigmented, and scaly erythematous rash. When this rash appears in a broad band around the neck, it is specifically referred to as **Casal’s necklace**. Pellagra is classically defined by the **"4 Ds"**: Dermatitis, Diarrhea, Dementia, and, if untreated, Death. **Analysis of Incorrect Options:** * **Scurvy (Vitamin C deficiency):** Characterized by defective collagen synthesis leading to "woody" edema, corkscrew hair, and gingival bleeding, but not a necklace-pattern rash. * **Kwashiorkor (Protein deficiency):** Presents with "flaky paint" dermatosis, edema, and "flag sign" in hair, but lacks the specific photosensitive distribution of Pellagra. * **Indicanuria (Hartnup Disease):** While Hartnup disease can *cause* Pellagra-like symptoms due to impaired tryptophan absorption, "Indicanuria" itself refers to the presence of indoxyl sulfate in urine (Blue Diaper Syndrome) and is not the name of the clinical condition presenting with the necklace. **High-Yield NEET-PG Pearls:** * **Precursor:** 60 mg of Tryptophan is required to synthesize 1 mg of Niacin. * **Associated Conditions:** Pellagra can be secondary to **Carcinoid Syndrome** (tryptophan diverted to serotonin) or **Hartnup Disease**. * **Dietary Link:** Maize-based diets are pellagragenic because niacin in maize is bound (**Niacytin**) and unavailable for absorption. * **Coenzymes:** Niacin is essential for **NAD+ and NADP+**, which are involved in redox reactions.

Question 468: Which of the following statements is true about Folic acid?

A. Is also called pteroyl glutamic acid
B. Is useful in the carriage of one-carbon atom moieties
C. Tetrahydrofolate is the active form
D. All of the above (Correct Answer)

Explanation: **Explanation:** Folic acid (Vitamin B9) is a crucial water-soluble vitamin essential for DNA synthesis and amino acid metabolism. This question tests the fundamental biochemical structure and function of the folate cycle. * **Option A (Structure):** Folic acid is chemically known as **Pteroylglutamic acid (PGA)**. It consists of three components: a Pteridine ring, Para-aminobenzoic acid (PABA), and Glutamic acid residues. * **Option B (Function):** The primary role of folate is the **transfer of one-carbon (1-C) units**. It acts as a carrier for moieties such as methyl (-CH₃), methylene (-CH₂-), methenyl (=CH-), formyl (-CHO), and formimino (-CH=NH) groups. These are vital for the synthesis of purines and thymidine (DNA synthesis). * **Option C (Active Form):** Dietary folate or synthetic folic acid is biologically inactive. It must be reduced by the enzyme **Dihydrofolate Reductase (DHFR)**, using NADPH, into its active coenzyme form: **Tetrahydrofolate (THF/FH4)**. Since all three statements are biochemically accurate, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **FIGLU Excretion Test:** Histidine metabolism requires THF. In folate deficiency, Formiminoglutamic acid (FIGLU) cannot be converted to glutamate and is excreted in the urine (a diagnostic marker). * **Neural Tube Defects (NTDs):** Periconceptional folic acid supplementation (400 µg/day) significantly reduces the risk of Spina Bifida and Anencephaly. * **Drug Interactions:** **Methotrexate** (anticancer drug) competitively inhibits Dihydrofolate Reductase, preventing the formation of THF. * **Megaloblastic Anemia:** Folate deficiency leads to impaired DNA synthesis, causing "macrocytic" RBCs and hypersegmented neutrophils. Unlike B12 deficiency, it does **not** cause neurological symptoms.

Question 469: Vitamin C is required for which of the following processes?

A. Posttranslational modification of collagen
B. Synthesis of epinephrine
C. Metabolism of tyrosine
D. All of the above (Correct Answer)

Explanation: **Explanation:** Vitamin C (Ascorbic Acid) acts as a potent reducing agent and a co-factor for several hydroxylase enzymes, maintaining metal ions (like $Fe^{2+}$ and $Cu^+$) in their reduced states. 1. **Posttranslational modification of collagen:** Vitamin C is a co-factor for **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes hydroxylate proline and lysine residues in procollagen, which is essential for the cross-linking and triple-helix stability of collagen fibers. 2. **Synthesis of epinephrine:** It is required by the enzyme **Dopamine $\beta$-hydroxylase**, which converts dopamine to norepinephrine. This is a crucial step in the catecholamine synthesis pathway occurring in the adrenal medulla. 3. **Metabolism of tyrosine:** Vitamin C is essential for the enzyme **p-hydroxyphenylpyruvate hydroxylase**, which converts p-hydroxyphenylpyruvate to homogentisic acid in the tyrosine catabolic pathway. **Why "All of the above" is correct:** Since Vitamin C is an indispensable co-factor for hydroxylation reactions in collagen synthesis, catecholamine production, and amino acid metabolism, all three processes are dependent on it. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency leads to defective collagen synthesis, characterized by "corkscrew hair," easy bruising, petechiae, and **bleeding gums**. * **Iron Absorption:** Vitamin C enhances the absorption of non-heme iron from the gut by reducing $Fe^{3+}$ (ferric) to $Fe^{2+}$ (ferrous). * **Bile Acid Synthesis:** It is a co-factor for **7-$\alpha$-hydroxylase**, the rate-limiting step in converting cholesterol to bile acids. * **Antioxidant Role:** It regenerates the reduced form of Vitamin E.

Question 470: Pellagra is primarily associated with a diet heavily reliant on which staple food?

A. Wheat
B. Rice
C. Maize (Correct Answer)
D. Milk

Explanation: **Explanation:** Pellagra is a clinical deficiency syndrome caused by a lack of **Niacin (Vitamin B3)** or its precursor amino acid, **Tryptophan**. **Why Maize is the Correct Answer:** Maize (corn) is associated with Pellagra for two primary reasons: 1. **Low Tryptophan:** Maize protein (zein) is significantly deficient in Tryptophan, which the body normally converts into Niacin (60 mg Tryptophan = 1 mg Niacin). 2. **Bound Niacin (Niacytin):** The niacin present in maize is chemically bound to complex carbohydrates and peptides, making it biologically unavailable for absorption in the gut. *Note: Traditional "Nixtamalization" (soaking corn in lime) releases this bound niacin, which is why Pellagra is rare in populations that process maize this way.* **Analysis of Incorrect Options:** * **Wheat & Rice:** These are staple cereals but contain sufficient available niacin and tryptophan to prevent clinical Pellagra. However, polished rice is specifically associated with **Beriberi** (Vitamin B1 deficiency). * **Milk:** Milk is actually a "Pellagra-preventive" food. Although it is low in preformed Niacin, it is exceptionally rich in **Tryptophan**, which compensates for the lack of the vitamin. **High-Yield Clinical Pearls for NEET-PG:** * **The 4 D’s of Pellagra:** Dermatitis (Casal’s necklace distribution), Diarrhea, Dementia, and Death. * **Hartnup Disease:** An autosomal recessive disorder involving defective transport of neutral amino acids (Tryptophan), leading to pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause secondary Pellagra because dietary tryptophan is diverted toward the overproduction of Serotonin. * **Jowar (Sorghum):** Also associated with Pellagra due to high **Leucine** content, which inhibits the enzyme *Quinolinate Phosphoribosyl Transferase (QPRT)* in the niacin synthesis pathway.

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

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Vitamin B6 and Transamination

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Folate and Vitamin B12 in One-Carbon Metabolism

Practice Questions

Vitamin C and Collagen Synthesis

Practice Questions

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