Vitamins and Coenzymes — MCQs
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Which of the following is not seen in Vitamin D deficiency?
Which of the following retinoids plays a major role in epithelial differentiation?
Which of the following is elevated in Pyridoxal Phosphate deficiency?
Poor wound healing in vitamin C deficiency is due to all of the following except:
Keshan disease is associated with cardiomegaly due to deficiency of which micronutrient?
All of the following are manifestations of hypervitaminosis A except?
Vitamin K is a cofactor in which of the following biochemical reactions?
Acrodermatitis enteropathica is due to a deficiency of which essential nutrient?
Who published an exhaustive treatise on scurvy?
Riboflavin is a constituent of which of the following?
Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs
Question 111: Which of the following is not seen in Vitamin D deficiency?
- A. Increased alkaline phosphatase
- B. Decreased phosphate in urine (Correct Answer)
- C. Hypophosphatemia
- D. Decreased serum calcium
Explanation: In Vitamin D deficiency (Rickets/Osteomalacia), the primary defect is the failure of calcium and phosphate absorption from the gut. This triggers a compensatory sequence of events that defines the biochemical profile of the disease. ### **Why "Decreased phosphate in urine" is the correct answer:** In Vitamin D deficiency, serum calcium levels fall. This stimulates the Parathyroid glands to secrete **Parathyroid Hormone (PTH)**—a condition known as **Secondary Hyperparathyroidism**. PTH acts on the proximal renal tubules to **inhibit phosphate reabsorption**, leading to **Phosphaturia** (increased phosphate in the urine). Therefore, phosphate in the urine is **increased**, not decreased. ### **Analysis of Incorrect Options:** * **A. Increased alkaline phosphatase:** This is the **earliest biochemical marker**. Low Vitamin D leads to defective mineralization; osteoblasts increase their activity to compensate, releasing ALP into the serum. * **C. Hypophosphatemia:** Due to the action of PTH (which causes renal phosphate wasting), serum phosphate levels drop significantly. * **D. Decreased serum calcium:** While PTH tries to maintain calcium by mobilizing it from bones, in chronic Vitamin D deficiency, the gut absorption is so poor that serum calcium eventually falls or remains at the lower limit of normal. ### **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Hallmarks:** ↓ Serum Calcium, ↓ Serum Phosphate, **↑ Serum ALP**, and **↑ Serum PTH**. * **Radiological Signs:** Cupping, splaying, and fraying of metaphyses (Rickets); Looser’s zones/Pseudofractures (Osteomalacia). * **Key Enzyme:** 1-alpha-hydroxylase in the kidney converts 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (Calcitriol). This enzyme is stimulated by PTH.
Question 112: Which of the following retinoids plays a major role in epithelial differentiation?
- A. Retinal
- B. Retinol
- C. Retinoic acid (Correct Answer)
- D. None of the above
Explanation: ### Explanation **Correct Answer: C. Retinoic Acid** **Mechanism of Action:** Retinoic acid functions as a hormone-like signaling molecule. It enters the nucleus and binds to specific nuclear receptors: **RAR (Retinoic Acid Receptor)** and **RXR (Retinoid X Receptor)**. These receptors bind to DNA sequences called **RAREs (Retinoic Acid Response Elements)**, acting as transcription factors that regulate the expression of genes responsible for cell growth and differentiation. In epithelial tissues, retinoic acid promotes the synthesis of specific keratins and mucus, preventing keratinization (squamous metaplasia) and maintaining the integrity of mucosal surfaces. **Why other options are incorrect:** * **Retinal (Retinaldehyde):** This is the aldehyde form of Vitamin A. Its primary role is in the **visual cycle**. It combines with the protein opsin to form rhodopsin, which is essential for vision in dim light. * **Retinol:** This is the transport and storage form (as retinyl esters) of Vitamin A. While it is the precursor to both retinal and retinoic acid, it does not directly mediate gene transcription or epithelial differentiation. **High-Yield NEET-PG Clinical Pearls:** * **Visual Cycle:** 11-cis retinal is the specific isomer required for vision; it is converted to all-trans retinal upon light exposure. * **Therapeutic Use:** All-trans retinoic acid (**Tretinoin**) is used in the treatment of **Acute Promyelocytic Leukemia (APL)** because it induces the differentiation of leukemic promyelocytes. * **Deficiency:** Vitamin A deficiency leads to **Bitot’s spots**, Xerophthalmia, and **Follicular Hyperkeratosis** (phrynoderma) due to the loss of epithelial differentiation. * **Teratogenicity:** Retinoids are highly teratogenic; pregnancy must be excluded before prescribing isotretinoin for acne.
Question 113: Which of the following is elevated in Pyridoxal Phosphate deficiency?
- A. FIGLU
- B. Homocystine
- C. Methylmalonic acid
- D. Xanthurenic acid (Correct Answer)
Explanation: **Explanation:** **Correct Option: D. Xanthurenic acid** Pyridoxal Phosphate (PLP), the active form of Vitamin B6, is a crucial cofactor in the **Kynurenine pathway** of Tryptophan metabolism. Specifically, the enzyme **Kynureninase** requires PLP to convert 3-hydroxykynurenine into 3-hydroxyanthranilic acid. In B6 deficiency, this pathway is blocked, leading to the shunting of metabolites toward the formation of **Xanthurenic acid**, which is then excreted in excess in the urine. This "Tryptophan Load Test" was historically used to diagnose B6 deficiency. **Analysis of Incorrect Options:** * **A. FIGLU (Formiminoglutamic acid):** Elevated in **Folic acid (B9)** deficiency. FIGLU is an intermediate in Histidine catabolism; its conversion to Glutamate requires Tetrahydrofolate. * **B. Homocystine:** While B6 is involved in converting Homocysteine to Cysteine (via Cystathionine $\beta$-synthase), an elevation in *Homocystine* (the oxidized dimer) is more classically associated with **Vitamin B12 or B9** deficiency or genetic Homocystinuria. However, Xanthurenic acid is the more specific marker for B6 deficiency in this context. * **C. Methylmalonic acid (MMA):** Elevated specifically in **Vitamin B12** deficiency. B12 is the cofactor for Methylmalonyl-CoA mutase; in its absence, MMA accumulates. **NEET-PG High-Yield Pearls:** * **Sideroblastic Anemia:** B6 is a cofactor for **ALA synthase** (the rate-limiting step of heme synthesis). Deficiency leads to microcytic anemia with ringed sideroblasts. * **Drug-Induced Deficiency:** **Isoniazid (INH)** therapy for TB induces B6 deficiency by forming inactive pyridoxal-hydrazones. Always co-administer B6 with INH. * **Neurotoxicity:** PLP is essential for the synthesis of GABA (inhibitory neurotransmitter). Deficiency can cause seizures, especially in infants.
Question 114: Poor wound healing in vitamin C deficiency is due to all of the following except:
- A. Inhibition of collagen synthesis (Correct Answer)
- B. Defective collagen synthesis
- C. Defective post-translational modification of collagen
- D. Defective hydroxylation
Explanation: **Explanation:** The core concept in Vitamin C (Ascorbic acid) deficiency (Scurvy) is not the **absence** of collagen production, but the production of **defective** collagen. **Why Option A is the Correct Answer:** Vitamin C is not required for the initial synthesis of the polypeptide chains (pre-procollagen) on the ribosomes. Therefore, collagen synthesis is **not inhibited**; rather, the collagen produced is structurally unstable. Because the question asks for the "except" factor, "Inhibition of collagen synthesis" is the correct choice as it does not occur. **Analysis of Incorrect Options:** * **B & C (Defective synthesis/Post-translational modification):** Vitamin C acts as a co-factor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes modify the collagen chain *after* it has been translated. Without Vitamin C, these modifications fail, leading to "defective" collagen that cannot form a stable triple helix. * **D (Defective hydroxylation):** This is the specific biochemical mechanism. Vitamin C maintains the iron cofactor of hydroxylase enzymes in the reduced (**Ferrous/Fe²⁺**) state. Lack of Vitamin C leads to failure of hydroxylation of proline and lysine residues. **NEET-PG High-Yield Pearls:** * **Scurvy Presentation:** Perifollicular hemorrhages, "corkscrew" hairs, swollen/bleeding gums, and subperiosteal hemorrhages. * **Enzyme Mechanism:** Vitamin C is essential for the **post-translational modification** of collagen in the Endoplasmic Reticulum. * **Stability:** Hydroxyproline is critical for the interchain hydrogen bonding that stabilizes the collagen triple helix. Without it, collagen denatures at normal body temperature.
Question 115: Keshan disease is associated with cardiomegaly due to deficiency of which micronutrient?
- A. Zinc
- B. Copper
- C. Selenium (Correct Answer)
- D. Iron
Explanation: **Explanation:** **Keshan disease** is a juvenile cardiomyopathy characterized by cardiomegaly, congestive heart failure, and cardiac arrhythmias. It is caused by a deficiency of **Selenium**, often exacerbated by a viral trigger (typically Coxsackievirus B). **Why Selenium is the Correct Answer:** Selenium is an essential trace element that functions as a vital cofactor for the enzyme **Glutathione Peroxidase**. This enzyme plays a critical role in the antioxidant defense system by neutralizing hydrogen peroxide and lipid hydroperoxides, thereby protecting cardiac myocytes from oxidative damage. In selenium-deficient individuals, increased oxidative stress leads to myocardial necrosis and subsequent fibrosis (cardiomegaly). **Analysis of Incorrect Options:** * **Zinc:** Deficiency typically leads to **Acrodermatitis enteropathica**, growth retardation, delayed wound healing, and hypogonadism. * **Copper:** Deficiency causes **Menkes Kinky Hair Syndrome**, microcytic anemia (refractory to iron), and neutropenia. Excess copper leads to Wilson’s disease. * **Iron:** Deficiency is the most common cause of **Microcytic Hypochromic Anemia**. While severe chronic anemia can cause high-output heart failure, it is not the primary cause of Keshan disease. **High-Yield Clinical Pearls for NEET-PG:** * **Kashin-Beck Disease:** Another selenium deficiency disorder characterized by osteoarthropathy (degeneration of joint cartilage). * **Selenocysteine:** Known as the **21st amino acid**, it is the form in which selenium is incorporated into proteins (e.g., Thioredoxin reductase, Deiodinases). * **Toxicity:** Excess selenium (Selenosis) causes garlic breath, hair loss (alopecia), and nail dystrophy.
Question 116: All of the following are manifestations of hypervitaminosis A except?
- A. Loss of hair
- B. Generalized exfoliation
- C. Decreased intracranial pressure (Correct Answer)
- D. Muscle pains
Explanation: ### Explanation Hypervitaminosis A (Vitamin A toxicity) occurs due to the excessive intake of preformed Vitamin A (retinoids). It can manifest as an acute or chronic condition, affecting multiple organ systems including the skin, musculoskeletal system, and central nervous system. **Why "Decreased intracranial pressure" is the correct answer:** Vitamin A toxicity is a classic cause of **Pseudotumor Cerebri** (Idiopathic Intracranial Hypertension). Instead of decreased pressure, it causes **increased intracranial pressure**. Clinical signs include headache, nausea, vomiting, and papilledema. In infants, this may manifest as a bulging fontanelle. **Analysis of Incorrect Options:** * **Loss of hair (Alopecia):** Chronic toxicity leads to thinning of hair and alopecia due to the effect of retinoids on the hair follicle cycle. * **Generalized exfoliation:** Vitamin A is essential for epithelial integrity. Excess levels lead to dry, pruritic skin and desquamation (exfoliation), often starting with the palms and soles. * **Muscle pains:** Hypervitaminosis A causes musculoskeletal symptoms, including bone pain, joint pain (arthralgia), and myalgia. It can also lead to cortical hyperostosis (excessive bone growth). --- ### NEET-PG High-Yield Pearls * **Teratogenicity:** Vitamin A is highly teratogenic (Category X). It can cause craniofacial anomalies and cardiac defects in the fetus. Pregnancy must be avoided during and shortly after Isotretinoin therapy. * **Acute Toxicity:** Can occur after a single massive dose (e.g., consuming polar bear liver). * **Carotenemia:** Excessive intake of Vitamin A precursors (Beta-carotene from carrots) causes yellowing of the skin but **does not** cause hypervitaminosis A or papilledema, as the conversion to retinol is regulated. * **Diagnosis:** Confirmed by elevated serum retinol levels (>100 µg/dL).
Question 117: Vitamin K is a cofactor in which of the following biochemical reactions?
- A. Carboxylation (Correct Answer)
- B. Hydroxylation
- C. Deamination
- D. Hydrolysis
Explanation: **Explanation:** **Vitamin K** (Phylloquinone/Menaquinone) serves as an essential cofactor for the enzyme **$\gamma$-glutamyl carboxylase**. This enzyme catalyzes the **post-translational carboxylation** of specific glutamate residues into **$\gamma$-carboxyglutamate (Gla)** on certain proteins. This process adds a second negative charge to the glutamate residue, allowing the protein to bind calcium ions ($Ca^{2+}$), which is essential for their functional activation. * **Why A is Correct:** Vitamin K is specifically required for the carboxylation of **Clotting Factors II, VII, IX, and X**, as well as Proteins C and S. Without this modification, these factors cannot bind to phospholipid surfaces, rendering them inactive. * **Why B is Incorrect:** Hydroxylation typically requires Vitamin C (e.g., prolyl hydroxylase in collagen synthesis) or Vitamin B3 (NADPH-dependent hydroxylases). * **Why C is Incorrect:** Deamination (removal of an amino group) involves enzymes like dehydratases or oxidases, often requiring Vitamin B6 (Pyridoxal Phosphate). * **Why D is Incorrect:** Hydrolysis is the cleavage of bonds by adding water, usually performed by digestive enzymes (proteases, lipases) which do not require Vitamin K. **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** Warfarin acts as a competitive inhibitor of **Vitamin K Epoxide Reductase (VKOR)**, preventing the regeneration of active Vitamin K (hydroquinone form), thereby inhibiting carboxylation. * **Osteocalcin:** Vitamin K is also a cofactor for the carboxylation of osteocalcin, a protein involved in bone mineralization. * **Newborns:** They are deficient in Vitamin K due to sterile guts and poor placental transfer; hence, a prophylactic IM injection of Vitamin K is given at birth to prevent **Hemorrhagic Disease of the Newborn**.
Question 118: Acrodermatitis enteropathica is due to a deficiency of which essential nutrient?
- A. Zinc (Correct Answer)
- B. Vitamin B3 (Niacin)
- C. Calcium
- D. Vitamin B6 (Pyridoxine)
Explanation: **Explanation:** **Acrodermatitis enteropathica (AE)** is a rare autosomal recessive disorder caused by a mutation in the **SLC39A4 gene**, which encodes the **ZIP4 transporter** responsible for zinc absorption in the duodenum and jejunum. Zinc is a vital trace element acting as a cofactor for over 300 enzymes (e.g., Alkaline Phosphatase, Carbonic Anhydrase). Deficiency leads to the classic triad of **periorificial and acral dermatitis, alopecia, and diarrhea**. Symptoms typically manifest in infants shortly after weaning from breast milk, as breast milk contains a zinc-binding ligand that facilitates absorption, unlike cow's milk. **Analysis of Incorrect Options:** * **Vitamin B3 (Niacin):** Deficiency causes **Pellagra**, characterized by the "3 Ds": Dermatitis (Casal’s necklace), Diarrhea, and Dementia. While it shares skin and GI symptoms, the distribution is typically in sun-exposed areas. * **Calcium:** Deficiency primarily leads to neuromuscular irritability (tetany, Chvostek/Trousseau signs) and bone pathologies (Rickets/Osteomalacia), not specific acral dermatitis. * **Vitamin B6 (Pyridoxine):** Deficiency presents with microcytic anemia, peripheral neuropathy, and seborrheic dermatitis, but is not the cause of AE. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Low serum zinc levels and **low serum Alkaline Phosphatase** (since ALP is a zinc-dependent enzyme). * **Treatment:** Lifelong high-dose oral zinc supplementation. * **Acquired Zinc Deficiency:** Can occur in patients on long-term Total Parenteral Nutrition (TPN) without trace element supplementation. * **Zinc & Immunity:** Zinc is essential for T-cell function; deficiency leads to thymic atrophy and poor wound healing.
Question 119: Who published an exhaustive treatise on scurvy?
- A. John Snow
- B. James Lind (Correct Answer)
- C. H. Khorana
- D. James Lister
Explanation: **Explanation:** The correct answer is **James Lind**. In 1753, James Lind, a Scottish naval surgeon, published *"A Treatise of the Scurvy."* He is historically significant for conducting one of the first ever clinical trials. He demonstrated that citrus fruits (lemons and oranges) could cure scurvy among sailors, identifying a dietary deficiency as the cause long before Vitamin C (ascorbic acid) was formally discovered. **Analysis of Incorrect Options:** * **John Snow:** Known as the "Father of Modern Epidemiology" for his work in tracing the source of a cholera outbreak in Soho, London (1854) to the Broad Street pump. * **H. Khorana (Har Gobind Khorana):** An Indian-American biochemist who won the Nobel Prize for his work on the interpretation of the genetic code and its function in protein synthesis. * **James Lister (Joseph Lister):** A pioneer of antiseptic surgery; he introduced the use of carbolic acid (phenol) to sterilize surgical instruments and clean wounds. **Clinical Pearls for NEET-PG:** * **Scurvy:** Caused by Vitamin C deficiency, leading to defective collagen synthesis (specifically failure of **hydroxylation of proline and lysine** residues). * **Key Symptoms:** Swollen/bleeding gums, perifollicular hemorrhages, "corkscrew" hairs, and poor wound healing. * **Biochemical Role:** Vitamin C acts as a reducing agent and is essential for the activity of **prolyl hydroxylase** and **lysyl hydroxylase** enzymes. It also enhances non-heme iron absorption in the gut.
Question 120: Riboflavin is a constituent of which of the following?
- A. Flavin mononucleotide (FMN) (Correct Answer)
- B. Nicotinamide adenine dinucleotide (NAD)
- C. Pyridoxal phosphate (PLP)
- D. Tetrahydrofolate (THF)
Explanation: **Explanation:** **Riboflavin (Vitamin B2)** is a water-soluble vitamin that serves as the precursor for two essential coenzymes: **Flavin Mononucleotide (FMN)** and **Flavin Adenine Dinucleotide (FAD)**. These coenzymes act as prosthetic groups for "flavoproteins," which are critical in oxidation-reduction reactions, particularly in the Mitochondrial Electron Transport Chain (Complex I and II) and the TCA cycle. **Analysis of Options:** * **Option A (Correct):** Riboflavin is phosphorylated by the enzyme ribokinase to form **FMN**. FMN can further be converted to FAD by the addition of an AMP moiety. * **Option B (Incorrect):** **NAD** (and NADP) are derived from **Niacin (Vitamin B3)**. They are involved in hydride ion transfer. * **Option C (Incorrect):** **PLP** is the active form of **Pyridoxine (Vitamin B6)**, primarily involved in transamination and decarboxylation reactions. * **Option D (Incorrect):** **THF** is the active form of **Folic Acid (Vitamin B9)**, essential for one-carbon metabolism and DNA synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Glutathione Reductase Test:** The activity of this enzyme in RBCs is the "gold standard" biochemical marker to assess Riboflavin status. * **Deficiency Manifestations:** Characterized by the "3 Cs": **C**heilosis (fissures at corners of mouth), **C**orneal vascularization, and **C**olored tongue (Magenta tongue). * **Light Sensitivity:** Riboflavin is highly sensitive to UV light; newborns undergoing phototherapy for neonatal jaundice may develop B2 deficiency and require supplementation.
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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