Folate and Vitamin B12 in One-Carbon Metabolism — MCQs

10 questions

Gastric secretions are essential for absorption of -

Which of the following is a consequence of Vitamin B12 deficiency?

30 years old came with complaints of easy fatigability, exertional dyspnea, and weight loss. She also complains of frequent falls. physical examination revealed there was a bilateral decrease in vibration sense. Her hemoglobin levels were 8.2g%. She was treated with folate. Her anemia improved but neurological symptoms worsened. Which of the following is the most probable reason for her condition?

Methyl-tetrahydrofolate (5-methyl-THF) gets accumulated in deficiency of which of the following?

All are cofactors for Dehydrogenase except:

Serine is converted to which amino acid during metabolism?

A 45-year-old patient presents with joint pain and weakness and is known to have homocystinuria. Which vitamin is required in the treatment?

Megaloblastic anemia should be treated with both folic acid and vitamin B12 because -

A patient is found to have elevated levels of methylmalonic acid. This finding suggests a deficiency in which vitamin?

Q10

Which vitamin is required for transfer of 1-carbon unit?

Folate and Vitamin B12 in One-Carbon Metabolism Indian Medical PG Practice Questions and MCQs

Question 1: Gastric secretions are essential for absorption of -

A. Cobalamin (Correct Answer)
B. Fat
C. Thiamine
D. Folic acid

Explanation: ***Cobalamin*** - **Intrinsic factor**, secreted by gastric parietal cells, is crucial for the absorption of **vitamin B12 (cobalamin)** in the terminal ileum [1]. - Without sufficient intrinsic factor, **pernicious anemia** can develop due to impaired B12 absorption [2]. *Fat* - Fat digestion primarily occurs in the **small intestine** with the help of **bile salts** and **pancreatic lipases**. - While gastric lipase begins some fat digestion, it's not essential for overall fat absorption. *Thiamine* - **Thiamine (vitamin B1)** is absorbed in the jejunum and ileum, primarily via **active transport** and passive diffusion. - Gastric secretions do not play a direct, essential role in its absorption. *Folic acid* - **Folic acid** is absorbed in the **duodenum and jejunum** as monoglutamates after being deconjugated from polyglutamate forms. - This process is not directly dependent on gastric secretions [2].

Question 2: Which of the following is a consequence of Vitamin B12 deficiency?

A. Enhanced folate absorption
B. Increased red blood cell production
C. Decreased metabolic intermediate levels
D. Neurological complications (Correct Answer)

Explanation: ### Neurological complications - **Vitamin B12** is crucial for maintaining the **myelin sheath** around nerves, and its deficiency leads to demyelination. - This can result in a range of neurological symptoms, including **peripheral neuropathy**, cognitive impairment, and **ataxia**. *Enhanced folate absorption* - **Vitamin B12 deficiency** actually impairs the proper utilization of **folate** within the cells, leading to a "folate trap" phenomenon [1]. - It does not enhance folate absorption; rather, it makes folate functionally deficient. *Increased red blood cell production* - **Vitamin B12 deficiency** leads to **megaloblastic anemia**, characterized by the production of **large, immature red blood cells** that are fewer in number [2]. - This results in a **decrease** in overall red blood cell production and an impaired ability to carry oxygen. *Decreased metabolic intermediate levels* - **Vitamin B12** is a cofactor for enzymes involved in critical metabolic pathways, such as the conversion of **methylmalonyl-CoA** to succinyl-CoA [1]. - Its deficiency leads to the **accumulation of these metabolic intermediates** (e.g., methylmalonic acid and homocysteine), not a decrease [1].

Question 3: 30 years old came with complaints of easy fatigability, exertional dyspnea, and weight loss. She also complains of frequent falls. physical examination revealed there was a bilateral decrease in vibration sense. Her hemoglobin levels were 8.2g%. She was treated with folate. Her anemia improved but neurological symptoms worsened. Which of the following is the most probable reason for her condition?

A. Folate therapy caused rapid use of Vit B12 stores aggravating symptoms (Correct Answer)
B. Vitamin B6 deficiency unmasked by folate treatment.
C. Impaired folate metabolism in the central nervous system.
D. Malabsorption of folate due to gastrointestinal issues.

Explanation: ***Folate therapy caused rapid use of Vit B12 stores aggravating symptoms*** - This patient likely has **Vitamin B12 deficiency**, which presents with **macrocytic anemia** and **neurological symptoms** like decreased vibration sense and frequent falls [1]. - While folate improves the anemia by allowing erythrocyte maturation, it can **deplete existing B12 stores** and worsen neurological symptoms in the presence of an underlying B12 deficiency [1]. *Vitamin B6 deficiency unmasked by folate treatment.* - **Vitamin B6 deficiency** can cause **microcytic anemia** and neuropathies, but it is not typically associated with the macrocytic anemia and specific neurological deterioration seen here after folate treatment. - Folate treatment itself does not directly unmask or worsen B6 deficiency in this manner. *Impaired folate metabolism in the central nervous system.* - Impaired folate metabolism in the CNS is rare and usually associated with specific genetic disorders or malformations, which does not explain the initial **macrocytic anemia** or the paradoxical worsening of neurological symptoms with folate. - The improvement in anemia with folate suggests that the central problem was not primarily impaired folate metabolism itself. *Malabsorption of folate due to gastrointestinal issues.* - If **folate malabsorption** was the primary issue, folate supplementation would not have improved the anemia, contrary to what is described in the case. - This option does not explain the worsening neurological symptoms after folate treatment.

Question 4: Methyl-tetrahydrofolate (5-methyl-THF) gets accumulated in deficiency of which of the following?

A. Vitamin B12 (Cobalamin) (Correct Answer)
B. Vitamin B2 (Riboflavin)
C. Vitamin B1 (Thiamine)
D. Vitamin B6 (Pyridoxine)

Explanation: ***Vitamin B12 (Cobalamin)*** - A deficiency in **Vitamin B12** (cobalamin) leads to the accumulation of **5-methyl-tetrahydrofolate (5-methyl-THF)** due to the **"folate trap"** hypothesis. - This occurs because B12 is a cofactor for **methionine synthase**, which converts 5-methyl-THF back to tetrahydrofolate (THF), a necessary step for DNA synthesis and other one-carbon metabolism reactions. - Without B12, folate remains trapped in the methyl form and cannot be utilized for other essential reactions. *Vitamin B2 (Riboflavin)* - **Vitamin B2** (riboflavin) is a precursor for **FAD** and **FMN**, essential coenzymes in various redox reactions in the electron transport chain and other metabolic pathways. - Its deficiency typically presents as **cheilosis**, glossitis, and angular stomatitis, but does not cause methyl-THF accumulation. *Vitamin B1 (Thiamine)* - **Vitamin B1** (thiamine) is a cofactor for enzymes like **pyruvate dehydrogenase** and **alpha-ketoglutarate dehydrogenase** in carbohydrate metabolism. - Its deficiency causes **Beriberi** (wet, dry, or Wernicke-Korsakoff syndrome), affecting the cardiovascular and nervous systems, but does not affect folate metabolism. *Vitamin B6 (Pyridoxine)* - **Vitamin B6** (pyridoxine) is a coenzyme for many metabolic reactions, particularly in amino acid metabolism and neurotransmitter synthesis. - Deficiency can lead to **sideroblastic anemia**, neurological symptoms, and dermatitis, but does not cause methyl-THF accumulation.

Question 5: All are cofactors for Dehydrogenase except:

A. SAM (Correct Answer)
B. NADP
C. NAD
D. FAD

Explanation: ***SAM*** - **S-adenosylmethionine (SAM)** is a cofactor involved in **methyl group transfer reactions**, carried out by enzymes known as methyltransferases. - Dehydrogenase enzymes catalyze **redox reactions**, typically involving the transfer of hydride ions, and thus do not utilize SAM as a cofactor. *NADP* - **Nicotinamide adenine dinucleotide phosphate (NADP)** is a crucial coenzyme for many **dehydrogenase reactions**, particularly in **anabolic pathways** like fatty acid synthesis and the pentose phosphate pathway. - It acts as an **electron carrier**, accepting or donating hydride ions. *NAD* - **Nicotinamide adenine dinucleotide (NAD)** is a highly common coenzyme for numerous **dehydrogenase enzymes**, especially in **catabolic pathways** such as glycolysis, the Krebs cycle, and oxidative phosphorylation. - It functions as an **electron acceptor** or donor in redox reactions. *FAD* - **Flavin adenine dinucleotide (FAD)** is a coenzyme derived from **riboflavin (Vitamin B2)** and is associated with various dehydrogenase enzymes, particularly those involved in **electron transport** and fatty acid oxidation. - FAD can accept two hydrogen atoms (one hydride and one proton) to become FADH₂.

Question 6: Serine is converted to which amino acid during metabolism?

A. Proline
B. Alanine
C. Glycine (Correct Answer)
D. None of the options

Explanation: ***Glycine*** - Serine is converted to **glycine** through a reaction catalyzed by **serine hydroxymethyltransferase**. - This reaction involves the transfer of a hydroxymethyl group from serine to **tetrahydrofolate**, forming **5,10-methylenetetrahydrofolate** and glycine. *Proline* - Proline is synthesized from **glutamate**, not directly from serine. - The pathway involves enzymes such as **glutamate 5-kinase** and **pyrroline-5-carboxylate reductase**. *Alanine* - Alanine is typically formed from **pyruvate** through **transamination reactions**, primarily by alanine transaminase. - Serine cannot be directly converted to alanine in a single metabolic step. *None of the options* - This option is incorrect because serine is indeed converted to glycine, a critical step in **one-carbon metabolism**.

Question 7: A 45-year-old patient presents with joint pain and weakness and is known to have homocystinuria. Which vitamin is required in the treatment?

A. Vitamin B6 (Correct Answer)
B. Vitamin B12
C. Vitamin B7
D. Vitamin B1
E. Vitamin B9

Explanation: ***Vitamin B6*** - **Homocystinuria** is often caused by a deficiency in the enzyme **cystathionine beta-synthase**, which requires **pyridoxal phosphate (active form of B6)** as a cofactor. - Supplementation with high-dose **vitamin B6** can help some patients by increasing the residual activity of the enzyme, thereby reducing **homocysteine levels**. - This is the **primary treatment** for **B6-responsive homocystinuria** (approximately 50% of cases respond to B6 therapy). *Vitamin B12* - Vitamin B12 is a cofactor for the enzyme **methionine synthase**, which converts homocysteine back to methionine. - While it plays a role in homocysteine metabolism, **vitamin B6** is typically the primary treatment for homocystinuria caused by **cystathionine beta-synthase deficiency**. *Vitamin B9* - Vitamin B9 (folic acid) works together with **vitamin B12** as a cofactor in the **remethylation pathway** via methionine synthase. - While folate supplementation may help lower homocysteine levels, it is **not the primary treatment** for classical homocystinuria due to cystathionine beta-synthase deficiency. - **Vitamin B6** remains the first-line vitamin therapy for enzyme deficiency-related homocystinuria. *Vitamin B7* - Vitamin B7, or **biotin**, is a cofactor for carboxylase enzymes and is involved in fatty acid synthesis and gluconeogenesis. - It has no direct role in the metabolism of **homocysteine** or the treatment of homocystinuria. *Vitamin B1* - Vitamin B1, or **thiamine**, is essential for carbohydrate metabolism and nerve function. - It is not involved in the metabolic pathways that regulate **homocysteine levels** or the treatment of homocystinuria.

Question 8: Megaloblastic anemia should be treated with both folic acid and vitamin B12 because -

A. It is an enzyme
B. It is a cofactor
C. Folic acid alone causes improvement of hematologic symptoms but worsening of neurological symptoms (Correct Answer)
D. None of the above

Explanation: ***Folic acid alone causes improvement of hematologic symptoms but worsening of neurological symptoms*** - Treating **B12 deficiency** with **folic acid alone** can normalize hematologic parameters, which can mask the underlying B12 deficiency. - This can lead to the progression of **irreversible neurological damage** because folic acid does not address the metabolic pathways dependent on B12 for myelin maintenance. *It is an enzyme* - **Folic acid** and **vitamin B12** are not enzymes; they function as **coenzymes** or **cofactors** in metabolic reactions. - Enzymes are proteins that catalyze biochemical reactions, a role not played by these vitamins themselves. *It is a cofactor* - While both **folic acid** and **vitamin B12** are cofactors, this statement alone does not explain why both are needed to treat megaloblastic anemia. - The critical reason for co-administration lies in the potential for **neurological deterioration** if B12 deficiency is missed and only folate is given. *None of the above* - This option is incorrect because there is a valid and specific reason (the neurological consequences mentioned above) for treating megaloblastic anemia with both supplements. - The interaction between **folate** and **B12 metabolism** is crucial in understanding the treatment approach.

Question 9: A patient is found to have elevated levels of methylmalonic acid. This finding suggests a deficiency in which vitamin?

A. Folate (important for DNA synthesis and red blood cell production).
B. Vitamin D (essential for calcium and phosphate homeostasis).
C. Vitamin B6 (involved in neurotransmitter synthesis and amino acid metabolism).
D. Vitamin B12 (required for methylmalonic acid metabolism). (Correct Answer)

Explanation: ***Vitamin B12 (required for methylmalonic acid metabolism).*** - An elevated level of **methylmalonic acid** (MMA) is a sensitive and specific indicator of **vitamin B12 deficiency** because vitamin B12 is a coenzyme for the enzyme **methylmalonyl-CoA mutase**, which converts methylmalonyl-CoA to succinyl-CoA. - Without sufficient **vitamin B12**, MMA accumulates and can be measured in serum or urine, leading to **neurological symptoms** and **megaloblastic anemia**. *Vitamin B6 (involved in neurotransmitter synthesis and amino acid metabolism).* - **Vitamin B6** (pyridoxine) is a cofactor for enzymes involved in amino acid metabolism, neurotransmitter synthesis, and **heme synthesis**, but it does not directly metabolize methylmalonic acid. - Deficiency in vitamin B6 can cause **sideroblastic anemia**, **neuropathy**, and **dermatitis**, not elevated MMA. *Folate (important for DNA synthesis and red blood cell production).* - **Folate** (vitamin B9) is crucial for DNA synthesis, red blood cell maturation, and amino acid metabolism, often leading to **megaloblastic anemia** when deficient. - Although folate deficiency can also cause **macrocytic anemia**, it does not lead to an accumulation of **methylmalonic acid**, distinguishing it from vitamin B12 deficiency. *Vitamin D (essential for calcium and phosphate homeostasis).* - **Vitamin D** is primarily involved in **calcium and phosphate homeostasis**, bone health, and immune function. - Deficiency in vitamin D can cause **rickets** in children and **osteomalacia** in adults, but it has no direct role in the metabolism of **methylmalonic acid**.

Question 10: Which vitamin is required for transfer of 1-carbon unit?

A. Niacin
B. Vitamin B12
C. Vitamin A
D. Folic acid (Correct Answer)

Explanation: ***Folic acid*** - **Folic acid** (vitamin B9) is essential for the transfer of **one-carbon units**, particularly as **tetrahydrofolate (THF)**. - These one-carbon units are critical in metabolic processes such as **DNA synthesis**, **amino acid metabolism**, and **neurotransmitter synthesis**. *Vitamin A* - **Vitamin A** (retinol) is primarily involved in **vision**, **immune function**, and **cell differentiation**. - It does not play a direct role in the transfer of one-carbon units. *Vitamin B12* - **Vitamin B12** (cobalamin) is involved in two main reactions: the conversion of **methylmalonyl-CoA to succinyl-CoA** and the transfer of a **methyl group from N5-methyl THF to homocysteine** to form methionine. - While it works with folate, it does not directly transfer one-carbon units in the same way as folic acid. *Niacin* - **Niacin** (vitamin B3) is a precursor to **NAD+ and NADP+**, which are crucial coenzymes in **redox reactions** and energy metabolism. - It is not involved in the transfer of one-carbon units.

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