NEET-PG 2020 - Biochemistry Practice Questions

Q: Enzyme activated by decrease in Insulin: glucagon ratio:

Glucose 6 phosphatase. ***Glucose 6 phosphatase*** - A decreased **insulin:glucagon ratio** signifies a catabolic state, promoting glucose release into the blood. - **Glucose-6-phosphatase** is the key enzyme in **gluconeogenesis** and **glycogenolysis** in the liver, dephosphorylating **glucose-6-phosphate** to **free glucose**, which can then be exported from the liver. *PFK* - **Phosphofructokinase (PFK)** is a key regulatory enzyme in **glycolysis**, which is inhibited in a state of low insulin:glucagon ratio. - Its activity would decrease, not increase, to reduce glucose utilization. *Glucokinase* - **Glucokinase** phosphorylates glucose to **glucose-6-phosphate** in the liver, trapping it for metabolism; its activity is increased by high insulin levels. - In a low insulin:glucagon ratio, its activity would be reduced to conserve glucose. *Hexokinase* - **Hexokinase** phosphorylates glucose in most peripheral tissues but has a lower Km for glucose than glucokinase, becoming saturated even at low glucose concentrations. - Its activity is not primarily regulated by the insulin:glucagon ratio; it is generally involved in glucose uptake for cellular energy needs.

Q: Which of the following is the rich source of Vitamin B12?

Meat. ***Meat*** - **Vitamin B12** is primarily found in **animal products** because it is synthesized by bacteria in the digestive tracts of animals. - **Meat**, especially red meat and liver, is an excellent source of this vitamin. *Mango* - Mangoes are a good source of **Vitamin C** and **Vitamin A**, but they contain negligible amounts of Vitamin B12. - As a fruit, mangos are a plant-based food and generally **do not contain B12**. *Carrot* - Carrots are rich in **beta-carotene** (a precursor to Vitamin A) and **fiber**, but they are not a source of Vitamin B12. - Being a vegetable, carrots are a **plant-based food** and lack Vitamin B12. *Spinach* - Spinach is known for its high content of **iron**, **folate**, and **Vitamin K**, but it does not contain Vitamin B12. - Like other plant-based foods, spinach **naturally lacks Vitamin B12**.

Q: NO is synthesized from:

L-arginine. ***L-arginine*** - **Nitric oxide (NO)** is synthesized from the amino acid **L-arginine** by the enzyme **nitric oxide synthase (NOS)**. - This reaction requires **molecular oxygen** and **NADPH** as cofactors, producing **L-citrulline** as a co-product. - **Three NOS isoforms** exist: neuronal (nNOS), inducible (iNOS), and endothelial (eNOS). - **Clinical significance**: NO is crucial for **vascular smooth muscle relaxation**, **neurotransmission**, and **immune defense**. *L-citrulline* - **L-citrulline** is the co-product (not precursor) of the NO synthesis reaction. - While it can be recycled back to **L-arginine** via the **citrulline-arginine cycle**, it does not directly yield NO without first being converted to arginine. - This recycling pathway helps sustain NO production in endothelial cells. *Glycine* - **Glycine** is involved in various metabolic processes, including **heme synthesis**, **purine synthesis**, and as a neurotransmitter. - It plays no direct role in **nitric oxide** biosynthesis. - Not a substrate for any **NOS isoform**. *Lysine* - **Lysine** is an essential amino acid important for **protein synthesis**, **collagen formation**, and **carnitine synthesis**. - It is structurally distinct from arginine and is not recognized by **nitric oxide synthase**. - Not involved in the NO synthesis pathway.

Q: If the percentage of thymine residues in DNA is 28%. What is the percentage of cytosine?

22%. ***22%*** - According to **Chargaff's rules**, in a DNA molecule, the amount of **adenine (A) is equal to the amount of thymine (T)**, and the amount of **guanine (G) is equal to the amount of cytosine (C)**. - If thymine (T) is 28%, then adenine (A) is also 28%, making a total of 56% for A+T. The remaining 44% (100% - 56%) is split equally between guanine and cytosine, so cytosine (C) is 22%. *36%* - This percentage would be plausible if the sum of adenine and thymine were 28%, which is incorrect as A and T are equal and their sum would thus be 56%. - This value does not adhere to the principle of **base pairing complementarity** and the total percentage of all bases summing to 100%. *44%* - This would be the combined percentage of guanine and cytosine, not the percentage of cytosine alone. - If cytosine were 44%, then guanine would also be 44%, leading to a total of 88% for G+C, which is inconsistent with T being 28%. *28%* - This is the percentage of thymine, and by **Chargaff's rules**, it would also be the percentage of adenine, not cytosine. - Cytosine percentages are derived from the remaining proportion of bases after accounting for adenine and thymine.

Q: A patient complains of knee pain. Routine investigations are unremarkable and still, the patient is unsatisfied. Urine turns black on standing, what is the enzyme involved?

Homogentisate oxidase. ***Homogentisate oxidase*** - **Homogentisate oxidase** deficiency leads to **alkaptonuria**, where homogentisic acid accumulates and is excreted in urine, turning it black upon standing or exposure to air. The knee pain could be due to **ochronosis**, the deposition of oxidized homogentisic acid in cartilage and connective tissues. - The unremarkable routine investigations with persistent knee pain suggest a less common metabolic disorder, which is consistent with **alkaptonuria**'s chronic and insidious presentation. *Methylmalonyl-CoA mutase* - A deficiency in **methylmalonyl-CoA mutase** causes **methylmalonic acidemia**, a metabolic disorder typically presenting in infancy with neurological symptoms, feeding difficulties, and developmental delay. - It does not cause urine to turn black on standing or lead to specific joint pain like that seen in ochronosis. *Xanthine oxidase* - **Xanthine oxidase** is involved in purine metabolism, and its inhibition (e.g., by allopurinol) is used to treat **gout**. - While gout can cause knee pain, it is characterized by elevated uric acid and typically responds to conventional treatments and the urine does not turn black. *Tyrosine hydroxylase* - **Tyrosine hydroxylase** is the rate-limiting enzyme in catecholamine synthesis, converting tyrosine to L-DOPA. - Defects in this enzyme are rare and result in neurological disorders, such as **DOPA-responsive dystonia**, and do not cause urine to blacken or lead to direct joint damage.

Q: Vitamin B12 is found maximum in:

Animal products. ***Animal products*** - **Vitamin B12**, or cobalamin, is primarily synthesized by bacteria and accumulates in animal tissues, making **meat, fish, poultry, eggs, and dairy** the richest dietary sources. - Humans generally cannot synthesize vitamin B12 and must obtain it from dietary sources, with **animal-derived foods** being the most concentrated and bioavailable forms. *Sunlight* - **Sunlight** is primarily critical for the synthesis of **Vitamin D** in the skin, not Vitamin B12. - Exposure to sunlight has no direct role in the production or absorption of **Vitamin B12**. *Green leafy vegetables* - **Green leafy vegetables** are excellent sources of nutrients like **folate, Vitamin K, and iron**, but they are generally devoid of naturally occurring **Vitamin B12**. - While some fortified plant-based foods may contain Vitamin B12, naturally, these vegetables do not provide it. *Roots and tubers* - **Roots and tubers** like potatoes, carrots, and sweet potatoes are good sources of carbohydrates, fiber, and various vitamins and minerals such as **Vitamin C and potassium**. - They do not contain significant amounts of **Vitamin B12**, as they are plant-based foods.

Q: Female, on maize as a staple diet. History of diarrhoea and lesion in the neck region. This is due to deficiency of which of the following?

Niacin. ***Niacin*** * The constellation of **diarrhea**, **dermatitis** (neck lesion, often a "Casal's necklace"), and the exclusive reliance on a **maize staple diet** is highly characteristic of **pellagra**, which is caused by a **niacin (vitamin B3) deficiency**. * Maize contains niacin in a bound form (niacytin) that is largely **unbioavailable** unless treated with alkali (e.g., historical limewater soaking in some cultures). *Pyridoxine* * Deficiency of **pyridoxine (vitamin B6)** can lead to symptoms such as **skin inflammation (seborrheic dermatitis)**, **glossitis**, **cheilosis**, and neurological issues like **peripheral neuropathy** or **seizures**. * It does not typically present with the classic **diarrhea** and **dermatitis** pattern associated with pellagra in the context of a maize-based diet. *Thiamine* * **Thiamine (vitamin B1) deficiency** is associated with **beriberi**, characterized by cardiovascular symptoms (**wet beriberi** with edema and heart failure) or neurological symptoms (**dry beriberi** with neuropathy and muscle wasting). * It does not cause the specific dermatological lesion on the neck or the triad of symptoms (dermatitis, diarrhea, and dementia) seen with niacin deficiency. *Zinc* * **Zinc deficiency** can manifest as **dermatitis (acrodermatitis enteropathica)**, **diarrhea**, **impaired immune function**, and **poor wound healing** or **growth retardation**. * While it can cause skin lesions and diarrhea, the distinct **photosensitive dermatosis** on the neck and the strong association with a maize staple diet specifically point away from zinc deficiency and towards pellagra.

Q: In a preterm baby with respiratory distress syndrome, which of the following lipids would be deficient?

Lecithin. ***Lecithin*** - **Lecithin** (also known as **phosphatidylcholine**) is the primary component of **surfactant** in the lungs, which reduces surface tension and prevents alveolar collapse. - In **preterm babies**, insufficient production of lecithin due to immature lung development leads to **respiratory distress syndrome (RDS)**. *Cardiolipin* - **Cardiolipin** is a major phospholipid found in the **inner mitochondrial membrane**, crucial for oxidative phosphorylation. - Deficiency is associated with mitochondrial disorders like **Barth syndrome**, not primary respiratory distress. *Sphingomyelin* - **Sphingomyelin** is a significant component of **cell membranes** and **myelin sheaths**, important for nerve insulation. - While present in the lungs, its primary role is not in surface tension reduction, and its deficiency is not directly linked to RDS. *Phosphatidylinositol* - **Phosphatidylinositol** is a precursor for various **signaling molecules** and plays a role in cell membrane structure. - While involved in cellular processes, it is not the critical surfactant component whose deficiency causes RDS.

Q: Which of the following is the etiology of Werner syndrome?

Short telomere with damaged DNA and loss of helicase. ***Short telomere with damaged DNA and loss of helicase*** - **Werner syndrome** is an **autosomal recessive disorder** characterized by **premature aging** due to mutations in the *WRN* gene, which codes for a **RecQ-type DNA helicase**. - The dysfunctional helicase leads to defects in **DNA replication, repair, and transcription**, resulting in **genomic instability**, **damaged DNA**, and **accelerated telomere attrition** (short telomeres). *Increased length of telomere* - **Increased telomere length** is generally associated with a **reduced rate of cellular aging** and is not characteristic of Werner syndrome or other premature aging disorders. - In most aging processes and syndromes like Werner, **telomeres tend to shorten** over time due to incomplete replication and oxidative stress. *Increased advanced glycation end products* - **Advanced glycation end products (AGEs)** accumulate in various tissues during normal aging and in conditions like diabetes, contributing to vascular and organ damage. - While AGEs play a role in the broader aging process, they are not the primary underlying genetic defect or direct etiology of Werner syndrome, which is a **DNA repair disorder**. *Decreased lipid peroxidation* - **Lipid peroxidation** is a process by which **free radicals** attack lipids, leading to cellular damage and is often associated with oxidative stress and aging. - A **decrease in lipid peroxidation** would generally be considered protective against aging-related damage, which is the opposite of what is seen in Werner syndrome, where there's an accelerated aging phenotype and increased cellular stress.

Q: Apolipoprotein B-48 is made by which process?

RNA editing. ***RNA editing*** - Apolipoprotein B-48 is synthesized from ApoB-100 mRNA through a process called **RNA editing** (specifically ApoB mRNA editing) - This involves a **cytidine deaminase enzyme (APOBEC-1)** that converts cytidine to uridine at position 6666, changing a glutamine codon (CAA) to a premature stop codon (UAA) in the small intestine - This results in a truncated protein that is 48% the length of ApoB-100 - ApoB-48 is produced in the **intestine**, while ApoB-100 (unedited) is produced in the **liver** *DNA editing* - DNA editing refers to permanent modifications in the DNA sequence itself - The ApoB gene remains unchanged; only the mRNA transcript is edited in intestinal cells - This is not the mechanism for producing ApoB-48 *RNA alternate splicing* - Alternative splicing involves selecting different combinations of exons from pre-mRNA to produce multiple mRNA isoforms - This process creates different protein variants through exon inclusion/exclusion - ApoB-48 production does not involve alternative splicing but rather direct nucleotide modification (C to U) within the coding sequence *RNA interference* - RNA interference (RNAi) is a biological process involving small RNA molecules (siRNA, miRNA) that silence gene expression - RNAi typically degrades mRNA or blocks translation - This process is not involved in generating a truncated protein like ApoB-48 from the same mRNA transcript

Question 1

Enzyme activated by decrease in Insulin: glucagon ratio:

Accepted Answer

Glucose 6 phosphatase

Answer

PFK

Answer

Glucokinase

Answer

Hexokinase

Question 2

Which of the following is the rich source of Vitamin B12?

Accepted Answer

Meat

Answer

Mango

Answer

Carrot

Answer

Spinach

Question 3

NO is synthesized from:

Accepted Answer

L-arginine

Answer

L-citrulline

Answer

Glycine

Answer

Lysine

Question 4

If the percentage of thymine residues in DNA is 28%. What is the percentage of cytosine?

Accepted Answer

22%

Answer

36%

Answer

44%

Answer

28%

Question 5

A patient complains of knee pain. Routine investigations are unremarkable and still, the patient is unsatisfied. Urine turns black on standing, what is the enzyme involved?

Accepted Answer

Homogentisate oxidase

Answer

Methylmalonyl-CoA mutase

Answer

Xanthine oxidase

Answer

Tyrosine hydroxylase

Question 6

Vitamin B12 is found maximum in:

Accepted Answer

Animal products

Answer

Sunlight

Answer

Green leafy vegetables

Answer

Roots and tubers

Question 7

Female, on maize as a staple diet. History of diarrhoea and lesion in the neck region. This is due to deficiency of which of the following?

Accepted Answer

Niacin

Answer

Pyridoxine

Answer

Thiamine

Answer

Zinc

Question 8

In a preterm baby with respiratory distress syndrome, which of the following lipids would be deficient?

Accepted Answer

Lecithin

Answer

Cardiolipin

Answer

Sphingomyelin

Answer

Phosphatidylinositol

Question 9

Which of the following is the etiology of Werner syndrome?

Accepted Answer

Short telomere with damaged DNA and loss of helicase

Answer

Increased length of telomere

Answer

Increased advanced glycation end products

Answer

Decreased lipid peroxidation

Question 10

Apolipoprotein B-48 is made by which process?

Accepted Answer

RNA editing

Answer

DNA editing

Answer

RNA alternate splicing

Answer

RNA interference

NEET-PG 2020 — Biochemistry