Vitamins and Coenzymes Practice Questions

Q: About scurvy, all are true EXCEPT:

Raised serum alkaline phosphatase. **Explanation:** Scurvy is caused by a deficiency of **Vitamin C (Ascorbic acid)**, which is essential for the hydroxylation of proline and lysine residues during collagen synthesis. Defective collagen leads to weakened connective tissue and fragile blood vessels. **Why Option C is the correct answer (The Exception):** In Scurvy, **Serum Alkaline Phosphatase (ALP) levels are typically low or normal**, not raised. ALP is an indicator of osteoblastic activity; in Vitamin C deficiency, osteoblastic function is suppressed because the formation of the osteoid matrix (which is primarily collagen) is impaired. This distinguishes Scurvy from Rickets, where ALP is characteristically elevated. **Analysis of Incorrect Options:** * **Option A (Subperiosteal hematomas):** Weakened capillary walls lead to bleeding under the periosteum. This causes intense pain and tenderness, often leading to "pseudoparalysis" where the child refuses to move the limbs. * **Option B (Separation of epiphysis):** Due to the failure of osteoid formation and a weakened metaphyseal zone (the "scorbutic lattice"), the epiphysis can easily displace or separate from the shaft. * **Option D (Gingival bleeding):** This is a classic hallmark of Scurvy. Defective collagen in the periodontal ligaments and capillary walls leads to swollen, "woody," and bleeding gums. **NEET-PG High-Yield Pearls:** * **Radiological Signs:** Look for **Frankel’s line** (dense zone of calcification), **Wimberger’s ring** (sclerotic margin around epiphysis), and **Pelkan spur** (metaphyseal osteophytes). * **Key Enzyme:** Vitamin C is a co-factor for **Prolyl hydroxylase** and **Lysyl hydroxylase**. * **Clinical Triad:** Anemia, Myalgia, and Dermatological signs (Perifollicular hemorrhages/Corkscrew hair).

Q: Which of the following are related to the functions of vitamin A?

All of the above. **Explanation:** Vitamin A (Retinol) is a fat-soluble vitamin that exists in three major active forms, collectively known as **retinoids**. Each form performs distinct and essential physiological functions in the body, making "All of the above" the correct choice. 1. **Retinol (Vitamin A Alcohol):** This is the primary storage and transport form of Vitamin A. It is essential for **reproduction** (spermatogenesis in males and preventing fetal resorption in females). 2. **Retinal (Vitamin A Aldehyde):** This form is crucial for the **visual cycle**. 11-cis-retinal combines with the protein opsin to form rhodopsin in the rod cells of the retina, which is necessary for vision in dim light. 3. **Retinoic Acid (Vitamin A Acid):** This form acts like a hormone. It binds to nuclear receptors (RAR/RXR) to regulate **gene expression**. It is essential for epithelial cell differentiation, growth, and mucus secretion. However, it *cannot* be converted back to retinal and thus cannot support vision or reproduction. **Why other options are incomplete:** While A, B, and C are all individual active forms, selecting only one would ignore the vital physiological roles played by the others. **High-Yield Clinical Pearls for NEET-PG:** * **Earliest Sign of Deficiency:** Nyctalopia (Night blindness). * **Earliest Objective Sign:** Conjunctival xerosis (followed by Bitot’s spots). * **Therapeutic Use:** All-trans retinoic acid (ATRA) is the drug of choice for **Acute Promyelocytic Leukemia (M3)**. * **Teratogenicity:** Isotretinoin (used for acne) is highly teratogenic; a pregnancy test is mandatory before prescription. * **Storage:** Primarily stored in the **Ito cells** (Stellate cells) of the liver.

Q: Which of the following fat-soluble vitamins functions as a coenzyme?

Vitamin K. **Explanation:** The correct answer is **Vitamin K**. While most fat-soluble vitamins (A, D, E, K) function primarily as hormones or antioxidants, **Vitamin K** is unique because it acts as an essential **coenzyme** for the enzyme **gamma-glutamyl carboxylase**. 1. **Why Vitamin K is correct:** Vitamin K (in its reduced form, hydroquinone) serves as a cofactor for the post-translational modification of specific proteins. It facilitates the **gamma-carboxylation of glutamate residues** on clotting factors **II, VII, IX, and X**, as well as Proteins C and S. This process adds a negative charge to the proteins, allowing them to bind calcium ions ($Ca^{2+}$) and attach to phospholipid membranes, which is critical for the coagulation cascade. 2. **Why other options are incorrect:** * **Vitamin A:** Functions primarily as a precursor to **retinal** (visual cycle) and **retinoic acid**, which acts like a hormone to regulate gene expression and epithelial differentiation. * **Vitamin D:** Functions as a **steroid hormone** (Calcitriol). It binds to nuclear receptors to regulate calcium and phosphate homeostasis. * **Vitamin E:** Functions primarily as a potent **lipid-soluble antioxidant**, protecting cell membranes from free radical damage (peroxidation); it has no known coenzyme function. **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K and thus inhibiting the activation of clotting factors. * **Newborns:** They are born with sterile guts and low Vitamin K stores, necessitating a prophylactic IM injection of Vitamin K to prevent **Hemorrhagic Disease of the Newborn**. * **Osteocalcin:** Vitamin K is also a coenzyme for the carboxylation of osteocalcin, playing a role in bone mineralization.

Q: In humans and other primates, as well as in guinea pigs, which vitamin cannot be synthesized due to the absence of the enzyme L-gulonolactone oxidase?

Ascorbic acid. **Explanation:** The correct answer is **Ascorbic acid (Vitamin C)**. **Why it is correct:** In most mammals, Vitamin C is synthesized from glucose via the **Uronic Acid Pathway**. The final step of this pathway involves the conversion of L-gulonolactone to 2-keto-L-gulonolactone, catalyzed by the enzyme **L-gulonolactone oxidase**. Humans, other primates, guinea pigs, and fruit bats lack the gene for this specific enzyme due to an evolutionary mutation. Consequently, they cannot synthesize Vitamin C endogenously and must obtain it through their diet. **Why the other options are incorrect:** * **Pantothenic acid (B5):** This is an essential vitamin obtained from various food sources (ubiquitous in nature). It is a precursor for Coenzyme A but is not synthesized via the uronic acid pathway. * **Nicotinic acid (B3):** While it is a vitamin, humans *can* synthesize a small amount of Niacin from the essential amino acid **Tryptophan** (60 mg Tryptophan = 1 mg Niacin). * **Folic acid (B9):** This is synthesized by plants and microorganisms. Humans lack the enzymatic machinery to couple the pteridine ring, PABA, and glutamic acid, making it an essential dietary requirement. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to defective collagen synthesis because it is a co-factor for **prolyl and lysyl hydroxylase** (post-translational modification). * **Reducing Agent:** Vitamin C maintains iron in the **ferrous (Fe²⁺) state**, facilitating dietary iron absorption in the duodenum. * **Antioxidant:** It acts as a potent water-soluble antioxidant and helps in the regeneration of Vitamin E. * **Uronic Acid Pathway:** While humans cannot make Vitamin C, the pathway is still functional for producing **UDP-glucuronate**, which is essential for the detoxification (conjugation) of bilirubin and various drugs.

Q: Which vitamin is required for glutathione to act as an antioxidant?

Niacin. **Explanation:** The correct answer is **Niacin (Vitamin B3)**. To understand why, we must look at the **Glutathione Redox Cycle**, which is the cell's primary defense against oxidative stress (Reactive Oxygen Species). **Why Niacin is Correct:** Glutathione exists in two forms: reduced (GSH) and oxidized (GSSG). To neutralize hydrogen peroxide, GSH is converted to GSSG by *Glutathione Peroxidase*. To remain effective, GSSG must be recycled back to GSH by the enzyme **Glutathione Reductase**. This enzyme requires **NADPH** as a mandatory co-factor to donate electrons. Niacin is the precursor for **NADP+/NADPH**. Without Niacin, NADPH levels drop, the recycling of glutathione fails, and the cell suffers oxidative damage. **Why Other Options are Incorrect:** * **Vitamin E:** While a potent antioxidant that protects lipid membranes, it does not serve as a co-factor for the enzymatic regeneration of glutathione. * **Vitamin C:** It works synergistically with Vitamin E and can help regenerate it, but it is not the direct co-enzyme required for the Glutathione Reductase reaction. * **Vitamin A:** Primarily involved in vision (retinal) and gene expression (retinoic acid); it does not play a direct role in the glutathione redox cycle. **Clinical Pearls for NEET-PG:** * **The HMP Shunt Connection:** The NADPH required for this process is primarily generated by the **Pentose Phosphate Pathway (HMP Shunt)** via the enzyme **G6PD**. * **G6PD Deficiency:** In G6PD deficiency, NADPH is low, leading to decreased reduced glutathione. This results in hemoglobin denaturation (forming **Heinz bodies**) and subsequent hemolysis. * **Selenium Connection:** While Niacin is the vitamin required, **Selenium** is the essential trace element required as a co-factor for *Glutathione Peroxidase*.

Q: What is one of the earliest symptoms of Vitamin A deficiency?

Nyctalopia. **Explanation:** **1. Why Nyctalopia is Correct:** Nyctalopia, or **night blindness**, is the earliest clinical manifestation of Vitamin A deficiency. Vitamin A (Retinol) is essential for the synthesis of **Rhodopsin** (visual purple) in the rod cells of the retina. Rhodopsin is responsible for vision in dim light. When Vitamin A levels drop, the regeneration of rhodopsin is impaired, leading to a decreased ability to see in low light. This is often the first symptom reported by the patient. **2. Why Other Options are Incorrect:** * **Xerophthalmia:** This is a spectrum of eye diseases caused by Vitamin A deficiency. While Nyctalopia is the first stage, Xerophthalmia refers to the progression into pathological dryness of the conjunctiva and cornea. * **Bitot’s Spots:** These are triangular, foamy, silvery-white patches on the bulbar conjunctiva. They represent keratinization of the conjunctiva and occur *after* night blindness has already manifested. * **Keratomalacia:** This is the **most severe** and late-stage manifestation. It involves liquefaction and necrosis of the cornea, leading to permanent blindness. **3. NEET-PG High-Yield Pearls:** * **WHO Classification of Xerophthalmia:** * **X1A:** Conjunctival xerosis * **X1B:** Bitot’s spots * **X2:** Corneal xerosis * **X3A/X3B:** Corneal ulceration/Keratomalacia * **Earliest Sign:** Conjunctival xerosis (objective sign). * **Earliest Symptom:** Nyctalopia (subjective complaint). * **Wald’s Visual Cycle:** Retinol is converted to 11-cis-retinal, which combines with opsin to form Rhodopsin. * **Treatment:** The standard WHO schedule for children >1 year is 200,000 IU of Vitamin A orally on days 0, 1, and 14.

Q: What is the active moiety of Coenzyme A?

Thiol group of pantetheine. **Explanation:** **1. Why the Correct Answer is Right:** Coenzyme A (CoA-SH) is a vital cofactor derived from Vitamin B5 (Pantothenic acid). Its primary function is to act as a carrier of acyl groups (like acetyl or succinyl) in metabolism. The "business end" or **active moiety** of the molecule is the **thiol (-SH) group**. This thiol group is specifically located at the end of the **cysteamine** residue, which, when combined with pantothenic acid, forms **pantetheine**. Acyl groups bind to this thiol group via a high-energy **thioester bond**, allowing them to be activated for various biochemical reactions like the TCA cycle and fatty acid synthesis. **2. Why the Incorrect Options are Wrong:** * **Option A (Acetyl group):** This is a substrate that binds to CoA to form Acetyl-CoA; it is not a structural part of the coenzyme itself. * **Option B (Pantothenic acid):** While Vitamin B5 is a structural precursor of CoA, it does not contain the reactive thiol group required for acyl group attachment. * **Option C (Thiol group of beta-alanine):** Beta-alanine is a component of pantothenic acid, but it does not possess a thiol group. The thiol group is provided by the amino acid **cysteine** (as cysteamine). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Components of CoA:** Adenosine 3',5'-bisphosphate + Pantothenic acid + Cysteamine. * **Rate-limiting step of CoA synthesis:** Phosphorylation of pantothenate by *pantothenate kinase*. * **High-energy bond:** The thioester bond in Acetyl-CoA is more energy-rich than an oxygen-ester bond, facilitating group transfer. * **Acyl Carrier Protein (ACP):** In fatty acid synthesis, the active moiety is also the 4'-phosphopantetheine group, similar to CoA.

Question 1

About scurvy, all are true EXCEPT:

Accepted Answer

Raised serum alkaline phosphatase

Answer

Subperiosteal hematomas with tenderness

Answer

Separation of epiphysis

Answer

Gingival bleeding

Question 2

Which of the following are related to the functions of vitamin A?

Accepted Answer

All of the above

Answer

Retinol

Answer

Retinal

Answer

Retinoic acids

Question 3

Which of the following fat-soluble vitamins functions as a coenzyme?

Accepted Answer

Vitamin K

Answer

Vitamin A

Answer

Vitamin E

Answer

Vitamin D

Question 4

Wernicke's encephalopathy, during refeeding syndrome, can be precipitated in deficiency of which of the following vitamin?

Accepted Answer

Thiamin

Answer

Riboflavin

Answer

Pyridoxine

Answer

Vitamin C

Question 5

In humans and other primates, as well as in guinea pigs, which vitamin cannot be synthesized due to the absence of the enzyme L-gulonolactone oxidase?

Accepted Answer

Ascorbic acid

Answer

Pantothenic acid

Answer

Nicotinic acid

Answer

Folic acid

Question 6

Which vitamin is required for glutathione to act as an antioxidant?

Accepted Answer

Niacin

Answer

Vitamin E

Answer

Vitamin C

Answer

Vitamin A

Question 7

Which form of vitamin D is measured in serum?

Accepted Answer

24,25-dihydroxyvitamin D

Answer

Cholecalciferol

Answer

25-hydroxyvitamin D

Answer

1,25-dihydroxyvitamin D

Question 8

What is one of the earliest symptoms of Vitamin A deficiency?

Accepted Answer

Nyctalopia

Answer

Xerophthalmia

Answer

Bitot's spot

Answer

Keratomalacia

Question 9

What is the active moiety of Coenzyme A?

Accepted Answer

Thiol group of pantetheine

Answer

Acetyl group

Answer

Pantothenic acid

Answer

Thiol group of beta-alanine

Question 10

Which of the following vitamins reduces the risk of insulin resistance, obesity, and the metabolic syndrome?

Accepted Answer

Vitamin D

Answer

Vitamin A

Answer

Vitamin C

Answer

Vitamin B12

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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