Vitamins and Coenzymes Practice Questions

Q: Which of the following statements about Vitamin B12 is incorrect?

It is required for the conversion of pyruvate to lactate.. **Explanation:** Vitamin B12 (Cobalamin) is a vital water-soluble vitamin that acts as a cofactor for only two specific enzymes in the human body. **Why Option D is the Correct Answer (The Incorrect Statement):** The conversion of **pyruvate to lactate** is catalyzed by the enzyme **Lactate Dehydrogenase (LDH)**. This reaction requires **Vitamin B3 (Niacin)** in the form of its coenzyme **NADH**, not Vitamin B12. Vitamin B12 has no role in anaerobic glycolysis or lactate metabolism. **Analysis of Incorrect Options (Correct Statements):** * **Option A:** Vitamin B12 has two active coenzyme forms: **Methylcobalamin** (found in the cytosol) and **Deoxyadenosylcobalamin** (found in the mitochondria). * **Option B:** Methylcobalamin is a cofactor for **Methionine Synthase**, which transfers a methyl group from methyl-tetrahydrofolate to homocysteine to form methionine. This is crucial for DNA synthesis. * **Option C:** Deoxyadenosylcobalamin is a cofactor for **Methylmalonyl-CoA Mutase**, which converts L-methylmalonyl-CoA to Succinyl-CoA, a key step in the catabolism of odd-chain fatty acids and certain amino acids. **High-Yield NEET-PG Clinical Pearls:** 1. **B12 Deficiency Markers:** Deficiency leads to an accumulation of both **Homocysteine** and **Methylmalonic Acid (MMA)**. In contrast, Folate deficiency only shows elevated Homocysteine. 2. **Neurological Symptoms:** Subacute Combined Degeneration (SCD) of the spinal cord occurs in B12 deficiency due to the failure of methylmalonyl-CoA metabolism, leading to abnormal myelin synthesis. 3. **Absorption:** Requires **Intrinsic Factor** (secreted by gastric parietal cells) and occurs in the **terminal ileum**.

Q: Which of the following plant parts does not contain keratin?

Potato. **Explanation:** The question contains a common terminology trap often encountered in biochemistry. In the context of plant nutrition, the term "keratin" is frequently used as a distractor for **Carotenoids** (specifically **Beta-carotene**). **1. Why Potato is the Correct Answer:** Potatoes are primarily composed of **starch** (amylopectin and amylose). While they contain small amounts of Vitamin C and B6, they are notably deficient in carotenoids (the precursors to Vitamin A). In biological terms, **Keratin** is a structural protein found in animals (hair, nails, skin) and is **never** found in any plant. However, in the context of this specific question type, it refers to the absence of yellow-orange pigments (carotenoids). **2. Analysis of Incorrect Options:** * **Carrot:** The richest source of $\beta$-carotene (the name "carotene" is derived from carrot). It is a potent provitamin A source. * **Spinach:** Although green due to chlorophyll, spinach is very high in $\beta$-carotene and lutein. The green pigment simply masks the orange carotenoids. * **Tomato:** Contains high levels of **Lycopene** and $\beta$-carotene, both of which belong to the carotenoid family. **3. NEET-PG High-Yield Pearls:** * **Vitamin A Precursor:** $\beta$-carotene is cleaved by the enzyme **$\beta$-carotene 15,15'-dioxygenase** in the intestinal mucosa to form two molecules of Retinal. * **Golden Rice:** A genetically modified variety of rice created to biosynthesize $\beta$-carotene to prevent Vitamin A deficiency. * **Clinical Correlation:** Excessive intake of the plants listed in options B, C, and D can lead to **Carotenemia** (yellowish discoloration of the skin), which is distinguished from jaundice by the **absence of scleral icterus**.

Q: Loose stool (diarrhea), skin rash (dermatitis), and memory loss (dementia) are characteristically seen in which of the following conditions?

Niacin deficiency. ### Explanation The correct answer is **Niacin deficiency (Option D)**. The clinical triad of **Diarrhea, Dermatitis, and Dementia** (the "3 Ds") is the hallmark of **Pellagra**, which results from a deficiency of Vitamin B3 (Niacin). If left untreated, it leads to the 4th "D"—Death. * **Dermatitis:** Characteristically presents as a symmetrical, scaly rash on sun-exposed areas, notably forming **Casal’s necklace** around the neck. * **Diarrhea:** Caused by atrophy of the columnar epithelium of the GI tract. * **Dementia:** Results from degeneration of neurons in the brain and spinal cord. **Analysis of Options:** * **Option A (Deficiency of Tryptophan):** While Niacin is synthesized from Tryptophan (60 mg Tryptophan = 1 mg Niacin), the question asks for the condition where these symptoms are *characteristically* seen. However, conditions like **Hartnup disease** (impaired tryptophan absorption) or **Carcinoid syndrome** (tryptophan diverted to serotonin) can lead to secondary Niacin deficiency and pellagra-like symptoms. * **Option B (Deficiency of Thiamine):** B1 deficiency leads to **Beriberi** (Dry: peripheral neuropathy; Wet: high-output heart failure) or **Wernicke-Korsakoff syndrome**, not the 3 Ds. * **Option C (Excess intake of Folic acid):** High folate intake is generally non-toxic but can mask a Vitamin B12 deficiency, leading to irreversible neurological damage. **High-Yield NEET-PG Pearls:** * **Coenzymes:** Niacin is a precursor for **NAD+ and NADP+**, essential for redox reactions. * **Corn/Maize Diet:** Populations dependent on corn are prone to Pellagra because corn is low in Tryptophan and its Niacin is in a bound, unabsorbable form (**Niacytin**). * **Key Enzyme:** Synthesis of Niacin from Tryptophan requires **Vitamin B6 (Pyridoxine)** as a cofactor. Thus, B6 deficiency can also manifest as Pellagra.

Q: Gamma-carboxylation of glutamic acid in clotting factors II, VII, and protein C is dependent on which vitamin?

Vitamin K. **Explanation:** **Vitamin K** is the essential cofactor for the enzyme **gamma-glutamyl carboxylase**. This enzyme catalyzes the post-translational modification of specific glutamic acid residues into **gamma-carboxyglutamic acid (Gla)**. This modification occurs in the liver for clotting factors **II, VII, IX, and X**, as well as anticoagulant **Proteins C and S**. The addition of a second carboxyl group creates a high-affinity binding site for **Calcium ions (Ca²⁺)**, which allows these proteins to bind to phospholipid membranes on platelets, a crucial step in the coagulation cascade. **Why other options are incorrect:** * **Vitamin C:** Acts as a cofactor for prolyl and lysyl hydroxylase, essential for **collagen synthesis**. Deficiency leads to Scurvy. * **Vitamin A:** Primarily involved in **vision** (retinal), gene transcription (retinoic acid), and epithelial maintenance. * **Vitamin E:** Functions as a potent **lipid-soluble antioxidant**, protecting cell membranes from free radical damage. **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** It inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the regeneration of active Vitamin K (hydroquinone), thereby inhibiting gamma-carboxylation. * **Newborns:** They are born with sterile guts and low Vitamin K stores; hence, a prophylactic **Vitamin K injection** is given at birth to prevent Hemorrhagic Disease of the Newborn. * **Gla Proteins:** Apart from clotting factors, **Osteocalcin** (bone protein) also undergoes Vitamin K-dependent gamma-carboxylation.

Q: A patient with chronic renal failure and bone pains will have a deficiency of which vitamin?

Vitamin D. **Explanation:** **Why Vitamin D is the Correct Answer:** The kidneys play a crucial role in the activation of Vitamin D. The final step of Vitamin D synthesis occurs in the proximal convoluted tubules of the kidney, where the enzyme **1-alpha-hydroxylase** converts 25-hydroxyvitamin D [25(OH)D] into its active form, **1,25-dihydroxyvitamin D (Calcitriol)**. In chronic renal failure (CRF), the loss of functional renal parenchyma leads to a deficiency of 1-alpha-hydroxylase. This results in decreased calcitriol levels, leading to impaired intestinal calcium absorption and hypocalcemia. To compensate, the parathyroid glands secrete excess PTH (**Secondary Hyperparathyroidism**), which mobilizes calcium from the bones, causing the "bone pains" and skeletal deformities known as **Renal Osteodystrophy**. **Why Other Options are Incorrect:** * **Vitamin A:** Primarily stored in the liver; its metabolism is not directly dependent on renal hydroxylation. Deficiency typically presents with night blindness (Nyctalopia), not bone pain. * **Vitamin B & C:** These are water-soluble vitamins. While they may be lost during dialysis, their primary deficiency is not a direct pathophysiological consequence of renal tissue failure itself. Vitamin C deficiency (Scurvy) causes gingival bleeding, while Vitamin B deficiencies present with neurological or hematological symptoms. **NEET-PG High-Yield Pearls:** * **Active form of Vit D:** Calcitriol (1,25-(OH)₂D₃). * **Storage form of Vit D:** Calcidiol (25-OH D₃) – this is what we measure to check for nutritional deficiency. * **Renal Osteodystrophy Triad:** Hyperphosphatemia (due to decreased excretion), Hypocalcemia, and Secondary Hyperparathyroidism. * **FGF-23:** A hormone that rises early in CRF to lower phosphate but also inhibits 1-alpha-hydroxylase, further worsening Vitamin D deficiency.

Q: All of the following vitamins are anti-oxidants EXCEPT:

Vitamin K. **Explanation** Antioxidants are substances that neutralize free radicals (Reactive Oxygen Species) to prevent cellular damage. In biochemistry, the primary antioxidant vitamins are **A, C, and E** (often remembered by the mnemonic **"ACE"**). **Why Vitamin K is the Correct Answer:** Vitamin K (Phylloquinone/Menaquinone) is primarily a co-factor for the **gamma-carboxylation of glutamate residues** on clotting factors II, VII, IX, and X. Its physiological role is centered on blood coagulation and bone metabolism, not the scavenging of free radicals. Therefore, it is not classified as an antioxidant. **Analysis of Other Options:** * **Beta-carotene (Option A):** A precursor to Vitamin A, it is a potent lipid-soluble antioxidant that quenches singlet oxygen and prevents lipid peroxidation in membranes. * **Ascorbic acid (Option B):** Vitamin C is the most important **water-soluble** antioxidant. It directly neutralizes ROS and is essential for regenerating the reduced (active) form of Vitamin E. * **Vitamin E (Option C):** Specifically $\alpha$-tocopherol, it is the most powerful **lipid-soluble** antioxidant. It protects LDL and polyunsaturated fatty acids (PUFAs) in cell membranes from oxidative damage. **High-Yield Clinical Pearls for NEET-PG:** * **Chain Breaker:** Vitamin E is known as the "Chain-breaking antioxidant" because it interrupts the self-propagating cycle of lipid peroxidation. * **Synergy:** Vitamin C and Vitamin E work in a redox cycle; Vitamin C acts as a reducing agent to "recharge" Vitamin E. * **Selenium Connection:** Selenium acts as a mineral antioxidant because it is a vital component of the enzyme **Glutathione Peroxidase**. * **Vitamin K Antagonist:** Warfarin inhibits Vitamin K Epoxide Reductase, preventing the recycling of Vitamin K.

Question 1

Deficiency of which vitamin affects tooth development?

Accepted Answer

Vitamin A

Answer

Carbohydrates

Answer

Vitamin E

Answer

Vitamin B

Question 2

Which of the following statements about Vitamin B12 is incorrect?

Accepted Answer

It is required for the conversion of pyruvate to lactate.

Answer

The active form is methylcobalamin.

Answer

It is required for the conversion of homocysteine to methionine.

Answer

It is required in the metabolism of methylmalonyl CoA.

Question 3

Which of the following plant parts does not contain keratin?

Accepted Answer

Potato

Answer

Tomato

Answer

Carrot

Answer

Spinach

Question 4

Loose stool (diarrhea), skin rash (dermatitis), and memory loss (dementia) are characteristically seen in which of the following conditions?

Accepted Answer

Niacin deficiency

Answer

Deficiency of Tryptophan

Answer

Deficiency of Thiamine

Answer

Excess intake of Folic acid

Question 5

Gamma-carboxylation of glutamic acid in clotting factors II, VII, and protein C is dependent on which vitamin?

Accepted Answer

Vitamin K

Answer

Vitamin C

Answer

Vitamin A

Answer

Vitamin E

Question 6

A patient with chronic renal failure and bone pains will have a deficiency of which vitamin?

Accepted Answer

Vitamin D

Answer

Vitamin A

Answer

Vitamin B

Answer

Vitamin C

Question 7

A 40-year-old male patient complains of diminished vision during night and dryness of eyes. The patient also has a history of recurrent bacterial infections. What will be the plasma level of retinol binding protein in this patient?

Accepted Answer

Decreased

Answer

Increased

Answer

Unchanged

Answer

Fluctuating

Question 8

All of the following vitamins are anti-oxidants EXCEPT:

Accepted Answer

Vitamin K

Answer

Beta-carotene

Answer

Ascorbic acid

Answer

Vitamin E

Question 9

A 45-year-old woman with longstanding Crohn disease and severe fat malabsorption experiences a fracture of the femoral neck after a minor contusion. This woman most likely has a deficiency of which of the following vitamins?

Accepted Answer

Vitamin D

Answer

Vitamin B1 (thiamine)

Answer

Vitamin C

Answer

Vitamin K

Question 10

Why does Vitamin B6 deficiency lead to anemia?

Accepted Answer

Heme synthesis is affected due to reduced activity of ALA synthase

Answer

Iron absorption is impaired

Answer

Red blood cell life span is reduced

Answer

Vitamin B6 deficiency causes bleeding tendencies

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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