Vitamins and Coenzymes Practice Questions

Q: Which of the following is not a component of coenzyme A?

Acetic acid. **Explanation:** Coenzyme A (CoA-SH) is a vital cofactor derived from Vitamin B5, primarily involved in the metabolism of fatty acids and the citric acid cycle. It functions as a carrier of acyl groups, forming high-energy thioester bonds. **Why Acetic Acid is the correct answer:** Acetic acid is **not** a structural component of Coenzyme A itself. Instead, acetic acid (as an acetyl group) binds to the pre-existing Coenzyme A molecule to form **Acetyl-CoA**. Acetic acid is a substrate that CoA carries, rather than a building block of the cofactor's structure. **Analysis of incorrect options:** * **Pantothenic Acid (Vitamin B5):** This is the core vitamin component of CoA. It is linked to beta-mercaptoethylamine and ADP. * **Adenylic Acid (Adenosine 3', 5'-bisphosphate):** CoA contains an adenine ring, a ribose sugar, and phosphate groups, which together constitute the adenylic acid portion of the molecule. * **Sulfhydryl Group (-SH):** This is the functional part of the molecule provided by **beta-mercaptoethylamine**. It is where acyl groups (like acetate) attach; hence CoA is often abbreviated as CoA-SH. **NEET-PG High-Yield Pearls:** 1. **Precursor:** Pantothenic acid (B5) is the essential vitamin precursor. Deficiency is rare but can cause "Burning Foot Syndrome." 2. **Functional Group:** The reactive part of CoA is the terminal **thiol (-SH) group**. 3. **Synthesis:** The rate-limiting step in CoA synthesis is the phosphorylation of pantothenate by **pantothenate kinase**. 4. **Key Derivatives:** Acetyl-CoA (2 carbons), Succinyl-CoA (4 carbons - used in heme synthesis), and HMG-CoA (cholesterol synthesis).

Q: Which vitamin is involved in one-carbon metabolism?

Folic acid. **Explanation:** **1. Why Folic Acid is Correct:** Folic acid (Vitamin B9) is the central coenzyme in **one-carbon metabolism**. Its active form, **Tetrahydrofolate (THF)**, acts as a carrier for one-carbon units (such as methyl, methylene, and formyl groups). These units are essential for the synthesis of purines and thymidine (DNA synthesis) and the conversion of homocysteine to methionine. Without THF, the cell cannot replicate its DNA or perform vital methylation reactions. **2. Why Other Options are Incorrect:** * **Thiamine (Vitamin B1):** Its active form, Thiamine Pyrophosphate (TPP), is involved in **oxidative decarboxylation** (e.g., Pyruvate Dehydrogenase complex) and the **transketolase** reaction in the HMP shunt. It does not carry one-carbon units. * **Pyridoxine (Vitamin B6):** Its active form, Pyridoxal Phosphate (PLP), is primarily involved in **transamination**, decarboxylation of amino acids, and heme synthesis. While it is a cofactor for *Serine Hydroxymethyltransferase* (which transfers a carbon to folate), PLP itself is not the one-carbon carrier. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Folate Trap":** A deficiency in Vitamin B12 leads to a functional folate deficiency because folate becomes "trapped" as N5-methyl-THF, unable to participate in DNA synthesis. * **Megaloblastic Anemia:** Deficiency of either Folate or B12 leads to impaired DNA synthesis, resulting in macrocytic cells. * **Neural Tube Defects (NTDs):** Maternal folate deficiency is a major risk factor; supplementation is recommended periconceptionally. * **Antimetabolites:** Drugs like **Methotrexate** inhibit *Dihydrofolate Reductase (DHFR)*, blocking the regeneration of THF and halting one-carbon metabolism.

Q: Select the TRUE statement about vitamin B12:

Requires a specific glycoprotein for its absorption. **Explanation:** **Why Option C is correct:** Vitamin B12 (Cobalamin) absorption is a complex physiological process. It requires **Intrinsic Factor (IF)**, a specific glycoprotein secreted by the **parietal cells** of the gastric mucosa. In the small intestine, the B12-IF complex binds to specific receptors in the **terminal ileum** for absorption. A deficiency of Intrinsic Factor (often due to autoimmune destruction of parietal cells) leads to **Pernicious Anemia**. **Analysis of Incorrect Options:** * **Option A:** Incorrect. Vitamin B12 acts as a vital cofactor for two key enzymes: **Methionine synthase** (converting homocysteine to methionine) and **Methylmalonyl-CoA mutase** (converting methylmalonyl-CoA to succinyl-CoA). * **Option B:** Incorrect. The transfer of amino groups (transamination) is the primary function of **Vitamin B6 (Pyridoxal Phosphate)**, not B12. * **Option D:** Incorrect. Vitamin B12 is synthesized exclusively by microorganisms. It is found naturally only in **animal-source foods** (meat, eggs, dairy). Strict vegetarians (vegans) are at high risk of deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Storage:** Unlike other water-soluble vitamins, B12 is stored in the **liver** in significant amounts (3–5 mg), enough to last 3–5 years. * **Metabolic Markers:** B12 deficiency leads to elevated levels of both **Homocysteine** and **Methylmalonic acid (MMA)**. (Note: Folate deficiency only elevates homocysteine). * **Neurological Impact:** Deficiency causes **Subacute Combined Degeneration (SCD)** of the spinal cord due to defective myelin synthesis. * **Transport:** In the blood, B12 is primarily bound to **Transcobalamin II**.

Q: A neonate is born with a gross deformity of her lower back. Examination of the subcutaneous lesion reveals disorganized neural tissue with entrapment of nerve roots. The parents are concerned about the risks for similar birth defects in their future offspring. You explain that supplementation of the maternal diet can reduce the incidence of neural tube defects. What is this important dietary supplement?

Folic acid. **Explanation:** The clinical presentation describes a **Neural Tube Defect (NTD)**, specifically **Myelomeningocele**, characterized by the protrusion of the spinal cord and meninges through a vertebral defect [1]. **1. Why Folic Acid is Correct:** Neural tube closure occurs between **days 21 and 28** of gestation (often before a woman knows she is pregnant). **Folic acid (Vitamin B9)** is a crucial co-enzyme for DNA synthesis and methylation [2]. Deficiency leads to impaired cell proliferation during the fusion of the neural folds. Supplementation significantly reduces the incidence of NTDs (like spina bifida and anencephaly) by up to 70%. **2. Why Other Options are Incorrect:** * **Niacin (B3):** Deficiency causes Pellagra (Dermatitis, Diarrhea, Dementia, Death). While it is vital for metabolic redox reactions, it is not specifically linked to neural tube closure. * **Thiamine (B1):** Deficiency leads to Beriberi or Wernicke-Korsakoff syndrome. It is essential for glucose metabolism but not primary neurulation. * **Vitamin B6 (Pyridoxine):** Used to treat pregnancy-induced nausea and as an adjunct in tuberculosis treatment (with Isoniazid), but it does not prevent NTDs. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dosage:** The standard dose for low-risk pregnancies is **400 mcg (0.4 mg)** daily [2]. For high-risk cases (previous child with NTD or mother on anticonvulsants like Valproate), the dose is **4 mg** daily. * **Timing:** Supplementation must begin **at least 1 month preconception** and continue through the first trimester. * **Screening:** Elevated **Alpha-fetoprotein (AFP)** in maternal serum and amniotic fluid is a marker for open NTDs [3]. Acetylcholinesterase levels in amniotic fluid are also increased. * **Mechanism:** Anticonvulsants (Valproate, Carbamazepine) and Methotrexate act as folic acid antagonists, increasing the risk of NTDs.

Q: What amount of tryptophan produces 1 gm of niacin?

60 mg. ### Explanation **Correct Answer: C. 60 mg** **1. Understanding the Concept:** Niacin (Vitamin B3) can be synthesized endogenously from the essential amino acid **Tryptophan** via the **Kynurenine pathway**. This process is relatively inefficient: it takes approximately **60 mg of dietary Tryptophan to produce 1 mg of Niacin**. This ratio is the basis for calculating "Niacin Equivalents" (NE) in nutrition. The conversion requires several cofactors, making it a high-yield topic for biochemistry: * **Vitamin B6 (Pyridoxine):** Required for the enzyme kynureninase. * **Vitamin B2 (Riboflavin):** Required for kynurenine hydroxylase. * **Iron:** Required for tryptophan pyrrolase. **2. Analysis of Options:** * **A, B, and D:** These values are incorrect. While individual metabolic rates may vary slightly, the internationally accepted nutritional standard for the conversion ratio is strictly **60:1**. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Pellagra:** A deficiency of Niacin characterized by the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Hartnup Disease:** An autosomal recessive disorder involving defective transport of neutral amino acids (including Tryptophan) in the gut and kidneys. This leads to a secondary Niacin deficiency and Pellagra-like symptoms. * **Carcinoid Syndrome:** In this condition, up to 60% of Tryptophan is diverted to synthesize **Serotonin** (5-HT), leaving insufficient amounts for Niacin synthesis, which can result in Pellagra. * **Corn/Maize Diets:** Diets based primarily on corn are pellagragenic because corn is low in Tryptophan and its Niacin is in a bound, unabsorbable form called **Niacytin**.

Q: Deficiency of which of the following vitamins is associated with progressive keratinization of the cornea?

Vitamin A. **Explanation:** **Vitamin A (Retinol)** is essential for maintaining the integrity of epithelial tissues throughout the body. In the eye, Vitamin A is required for the differentiation of conjunctival and corneal epithelial cells. Its deficiency leads to **squamous metaplasia**, where the normal mucus-secreting columnar epithelium is replaced by keratinized squamous epithelium. This process, known as **Xerophthalmia**, begins with conjunctival xerosis and progresses to **corneal keratinization (Xerosis iridis)**, Bitot’s spots, and eventually **Keratomalacia** (liquefactive necrosis of the cornea). **Analysis of Incorrect Options:** * **Vitamin C (Ascorbic Acid):** Primarily involved in collagen synthesis. Deficiency leads to **Scurvy**, characterized by capillary fragility, gingival bleeding, and impaired wound healing, but not corneal keratinization. * **Niacin (Vitamin B3):** Deficiency causes **Pellagra**, characterized by the "3 Ds": Dermatitis, Diarrhea, and Dementia. While it affects the skin, it does not cause primary corneal keratinization. * **Pyridoxine (Vitamin B6):** Functions as a cofactor in amino acid metabolism. Deficiency typically presents with peripheral neuropathy, sideroblastic anemia, and seborrheic dermatitis. **High-Yield Clinical Pearls for NEET-PG:** * **Earliest symptom** of Vitamin A deficiency: **Nyctalopia** (Night blindness). * **Earliest clinical sign**: **Conjunctival xerosis** (dryness). * **Bitot’s Spots:** Triangular, foamy patches on the bulbar conjunctiva due to keratin buildup and *Corynebacterium xerosis* gas production. * **WHO Classification:** Xerophthalmia is graded from X1A (Conjunctival xerosis) to X3B (Keratomalacia involving >1/3 of the cornea). * **Vitamin A** is also a component of **Rhodopsin** (visual purple) in retinal rods, essential for low-light vision.

Q: Pellagra can cause all of the following except?

Acute hemorrhagic disseminated encephalomyelitis. **Explanation:** Pellagra is caused by a deficiency of **Niacin (Vitamin B3)** or its precursor, the amino acid **Tryptophan**. It is classically characterized by the "4 Ds": **Dermatitis, Diarrhea, Dementia, and Death.** **Why Option D is correct:** **Acute Hemorrhagic Disseminated Encephalomyelitis (AHDE)** is a hyperacute, inflammatory demyelinating disease of the central nervous system, often considered a fulminant form of ADEM. It is typically post-infectious or autoimmune in etiology, not nutritional. Therefore, it is not a manifestation of Pellagra. **Why other options are incorrect:** * **Option A (Depression):** The "Dementia" component of Pellagra encompasses a wide range of neuropsychiatric symptoms, including irritability, depression, anxiety, and cognitive decline. * **Option B (Skin lesions):** Dermatitis is a hallmark of Pellagra. It typically presents as a symmetric, photosensitive erythematous rash, often forming **Casal’s necklace** around the neck. * **Option C (Spastic paretic syndrome):** In advanced or chronic cases, Pellagra can lead to neurological degeneration involving the pyramidal tracts and posterior columns of the spinal cord, resulting in spasticity and gait disturbances. **High-Yield Clinical Pearls for NEET-PG:** * **Hartnup Disease:** An autosomal recessive disorder involving defective neutral amino acid transport (Tryptophan), leading to pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause Pellagra because Tryptophan is diverted to produce excessive Serotonin instead of Niacin. * **Corn/Maize Diets:** Diets dependent on untreated maize can lead to Pellagra because the niacin is bound (**Niacytin**) and unavailable for absorption. * **Coenzyme forms:** Niacin is essential for the synthesis of **NAD+ and NADP+**, which are vital for redox reactions.

Q: Which of the following is true about scurvy?

Bowing of legs.. **Explanation:** Scurvy results from a deficiency of **Vitamin C (Ascorbic acid)**, which is a critical cofactor for the hydroxylation of proline and lysine residues during collagen synthesis. Defective collagen formation leads to impaired osteoid matrix production and fragile blood vessels. **1. Why Option C is Correct:** In pediatric scurvy (Barlow’s disease), the primary defect is in the **formation of the osteoid matrix**, not mineralization. While the cartilage calcifies normally, it cannot be replaced by bone. This leads to a weak, brittle primary spongiosa. Under the stress of weight-bearing, these weakened bones undergo microfractures and deformities, leading to **bowing of the legs** (similar to rickets, though the underlying mechanism differs). **2. Why the Other Options are Incorrect:** * **Option A:** Skeletal changes are predominantly seen in **growing children**. In adults, the skeletal system is already formed; therefore, scurvy manifests primarily as gingival hemorrhages, perifollicular petechiae, and impaired wound healing rather than bone deformities. * **Option B:** Defective mineralization is the hallmark of **Rickets/Osteomalacia** (Vitamin D deficiency). In Scurvy, mineralization is normal, but the **protein matrix (collagen)** is defective. * **Option D:** Widening of the metaphyseal plate in scurvy is due to the **persistence of calcified cartilage** (which cannot be resorbed or replaced by bone), not callus overgrowth. **High-Yield Clinical Pearls for NEET-PG:** * **Radiological Signs:** * **Frankel’s Line:** Dense zone of provisional calcification at the epiphysis. * **Wimberger’s Ring:** Dense shell of calcification around a lucent epiphysis. * **Pelkan Spur:** Marginal bony outgrowths at the metaphysis. * **Trummerfeld Zone:** A lucent "scurvy line" representing the scorbutic lattice. * **Key Pathology:** Subperiosteal hemorrhage is a classic finding due to capillary fragility.

Q: Conversion of glycine to serine requires which of the following?

Folic acid. **Explanation:** The conversion of **Glycine to Serine** is a reversible reaction catalyzed by the enzyme **Serine Hydroxymethyltransferase (SHMT)**. This reaction is a classic example of **one-carbon metabolism**. 1. **Why Folic Acid is Correct:** The enzyme SHMT requires **Tetrahydrofolate (THF)**, the active form of Folic acid, as a coenzyme. Specifically, it uses **N5, N10-methylene THF** to donate a hydroxymethyl group to glycine to form serine. Pyridoxal Phosphate (Vitamin B6) is also required as a cofactor for this reaction. 2. **Why Other Options are Incorrect:** * **Thiamine (B1):** Acts as a cofactor (TPP) for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase) and transketolase reactions, not one-carbon transfers. * **Vitamin C:** Primarily involved in collagen synthesis (hydroxylation of proline/lysine) and as an antioxidant; it does not participate in glycine metabolism. * **Iron (Fe2+):** Acts as a cofactor for enzymes like prolyl hydroxylase or in the electron transport chain, but is not involved in the SHMT reaction. **High-Yield Clinical Pearls for NEET-PG:** * **One-Carbon Pool:** Folic acid is the primary carrier of one-carbon units (formyl, methyl, methylene groups). * **The "Folate Trap":** A deficiency in Vitamin B12 leads to folate being trapped as N5-methyl THF, which impairs DNA synthesis and causes megaloblastic anemia. * **Glycine Cleavage System:** This is the major pathway for glycine degradation, which also requires THF and NAD+, producing CO2 and NH4+. Deficiency in this system leads to **Non-ketotic Hyperglycinemia**. * **Dual Cofactor Requirement:** Remember that SHMT is one of the few enzymes requiring both **Folate and B6 (PLP)**.

Q: Magenta red tongue with cheilosis is caused by deficiency of?

Riboflavin. **Explanation:** The correct answer is **Riboflavin (Vitamin B2)**. **1. Why Riboflavin is correct:** Riboflavin deficiency, also known as **ariboflavinosis**, characteristically manifests with mucocutaneous lesions. The classic triad includes **magenta-colored tongue** (glossitis), **cheilosis** (fissuring at the corners of the mouth), and **corneal neovascularization**. The magenta hue of the tongue is due to inflammation and atrophy of the lingual papillae, which allows the underlying vascularity to give it a distinct purplish-red appearance. **2. Why other options are incorrect:** * **Thiamine (B1):** Deficiency leads to **Beriberi** (Dry: peripheral neuropathy; Wet: high-output heart failure) or **Wernicke-Korsakoff syndrome**. It does not typically cause magenta tongue. * **Cyanocobalamin (B12):** Deficiency causes megaloblastic anemia and subacute combined degeneration of the spinal cord. While it causes glossitis, the tongue is usually described as **"beefy red"** and smooth (Hunter’s glossitis), not magenta. * **Ascorbic acid (C):** Deficiency leads to **Scurvy**, characterized by swollen/bleeding gums, perifollicular hemorrhages, and poor wound healing. **3. NEET-PG High-Yield Pearls:** * **Coenzyme forms:** Riboflavin is the precursor for **FMN** and **FAD**, which are essential for redox reactions (e.g., Succinate dehydrogenase in TCA cycle). * **Diagnostic Test:** The most sensitive functional test for B2 deficiency is the measurement of **Erythrocyte Glutathione Reductase activity**. * **The "C"s of B2:** **C**heilosis, **C**orneal neovascularization, and **C**onfused with B3 (Pellagra-like dermatitis). * **Note:** Riboflavin is light-sensitive; hence, newborns undergoing phototherapy for jaundice may develop B2 deficiency.

Question 1

Which of the following is not a component of coenzyme A?

Accepted Answer

Acetic acid

Answer

Pantothenic acid

Answer

Adenylic acid

Answer

Sulfhydryl group

Question 2

Which vitamin is involved in one-carbon metabolism?

Accepted Answer

Folic acid

Answer

Thiamine

Answer

Pyridoxine

Answer

All of the above

Question 3

Select the TRUE statement about vitamin B12:

Accepted Answer

Requires a specific glycoprotein for its absorption

Answer

Does not act as a cofactor

Answer

Involved in the transfer of amino groups

Answer

Dietary requirement is met by intake of plant products

Question 4

A neonate is born with a gross deformity of her lower back. Examination of the subcutaneous lesion reveals disorganized neural tissue with entrapment of nerve roots. The parents are concerned about the risks for similar birth defects in their future offspring. You explain that supplementation of the maternal diet can reduce the incidence of neural tube defects. What is this important dietary supplement?

Accepted Answer

Folic acid

Answer

Niacin

Answer

Thiamine

Answer

Vitamin B6

Question 5

What amount of tryptophan produces 1 gm of niacin?

Accepted Answer

60 mg

Answer

40 mg

Answer

50 mg

Answer

70 mg

Question 6

Deficiency of which of the following vitamins is associated with progressive keratinization of the cornea?

Accepted Answer

Vitamin A

Answer

Vitamin C

Answer

Niacin

Answer

Pyridoxine

Question 7

Pellagra can cause all of the following except?

Accepted Answer

Acute hemorrhagic disseminated encephalomyelitis

Answer

Depression

Answer

Skin lesions

Answer

Spastic paretic syndrome

Question 8

Which of the following is true about scurvy?

Accepted Answer

Bowing of legs.

Answer

Skeletal changes in adults occur with deficiency of vitamin C.

Answer

Defective mineralization is the central cause of bone changes.

Answer

Callus overgrowth results in widening of the metaphyseal plate.

Question 9

Conversion of glycine to serine requires which of the following?

Accepted Answer

Folic acid

Answer

Thiamine

Answer

Vitamin C

Answer

Iron (Fe2+)

Question 10

Magenta red tongue with cheilosis is caused by deficiency of?

Accepted Answer

Riboflavin

Answer

Thiamine

Answer

Cyanocobalamine

Answer

Ascorbic acid

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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