NEET-PG 2012 - Biochemistry Practice Questions

Q: What is the most sensitive biochemical marker for a 7-day old myocardial infarction?

Troponin I/T. ***Troponin I/T*** - **Cardiac troponins (I and T)** are highly sensitive and specific biomarkers for **myocardial injury**. - While they rise quickly after an MI, they also remain elevated for an extended period, typically **7 to 10-14 days**, making them ideal for detecting a 7-day-old event. *CPK MB* - **Creatine phosphokinase MB (CPK-MB)** is a well-known cardiac marker, but its elevation is more transient, usually returning to normal within **2-3 days** post-MI. - Therefore, it would likely be undetectable or near baseline 7 days after the event, making it insensitive for this duration. *LDH* - **Lactate dehydrogenase (LDH)** used to be used as a cardiac marker, but it is **non-specific** and found in various tissues. - While it can remain elevated for an extended period after an MI (up to 10-14 days), its lack of specificity makes other markers, particularly troponins, much more reliable for confirming myocardial damage. *Myoglobin* - **Myoglobin** is one of the earliest markers to rise after myocardial injury, but it is also **rapidly cleared** from the bloodstream, usually within 24 hours. - Due to its short half-life, myoglobin would not be elevated 7 days after an MI, making it unsuitable for detecting such a remote event.

Q: What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?

2 ATP, 2 NADH. **2 ATP, 2 NADH** - Glycolysis has a net yield of **2 molecules of ATP** because 4 ATP molecules are produced, but 2 ATP molecules are consumed during the initial energy investment phase. - **2 molecules of NADH** are also produced during the energy generation phase when glyceraldehyde-3-phosphate is oxidized. *4 ATP, 2 NADH* - While 4 ATP molecules are indeed produced during glycolysis, this option does not account for the **2 ATP molecules consumed** in the initial steps, leading to an incorrect net value. - The production of **2 NADH** is correct, but the ATP count is the gross rather than the net. *4 ATP, 4 NADH* - This option overstates the production of both ATP and NADH. While **4 ATP are produced (gross)**, the net is 2 ATP. - Only **2 NADH** molecules are formed per glucose molecule in glycolysis, not 4. *2 ATP, 4 NADH* - This option accurately reflects the **net ATP yield of 2 molecules**. - However, it exaggerates the production of NADH, as only **2 molecules of NADH** are formed during glycolysis, not 4.

Q: Which enzyme is required for cutting the DNA strand during synthesis?

Topoisomerase. ***Topoisomerase*** - **Topoisomerases** are enzymes essential for DNA replication; they induce temporary **single- or double-strand breaks** in DNA to relieve **supercoiling** ahead of the replication fork. - This cutting and rejoining activity prevents the DNA from becoming excessively tangled and facilitates the unwinding process required for synthesis. *DNA polymerase* - **DNA polymerase** is responsible for **synthesizing new DNA strands** by adding nucleotides, not for cutting the DNA backbone. - It works by moving along the template strand, reading the bases, and then adding complementary nucleotides to the growing DNA strand. *DNA ligase* - **DNA ligase** functions to **join DNA fragments** together by forming phosphodiester bonds, especially in sealing Okazaki fragments during lagging strand synthesis. - Its role is to ligate (join) cut strands, not to initiate cuts in the DNA. *Helicase* - **Helicase** unwinds the DNA double helix into single strands using ATP hydrolysis; it **separates the two strands** but does not cut the phosphodiester backbone. - This enzyme creates the replication fork by disrupting hydrogen bonds between base pairs, making the DNA accessible for replication machinery.

Q: Which of the following is not a component of a nucleotide?

Fatty acid. ***Fatty acid*** - A **fatty acid** is a component of **lipids**, such as triglycerides and phospholipids, which are structurally and functionally distinct from **nucleotides**. - **Nucleotides** are the building blocks of nucleic acids (DNA and RNA), whereas fatty acids are essential for cell membranes and energy storage. *Sugar* - A **pentose sugar** (either **deoxyribose** in DNA or **ribose** in RNA) is a fundamental component of every nucleotide. - This sugar forms the backbone of the nucleic acid strand, covalently linked to the phosphate group and the nitrogenous base. *Phosphate* - A **phosphate group** is a crucial component of a nucleotide, providing the negative charge and forming the phosphodiester bonds that link nucleotides together into a nucleic acid chain. - The number of phosphate groups (mono-, di-, or triphosphate) determines the nucleotide's energy state and function. *Base* - A **nitrogenous base** (adenine, guanine, cytosine, thymine, or uracil) is an essential component of a nucleotide, responsible for genetic information storage and pairing. - This base is attached to the pentose sugar and determines the specific identity of the nucleotide within the DNA or RNA sequence.

Q: Hydrolysis occurs at which step of urea cycle ?

Cleavage of arginine. ***Cleavage of arginine*** - The final step in the urea cycle, where **arginine** is hydrolyzed by the enzyme **arginase** to form **urea** and **ornithine**. - This reaction involves the addition of a **water molecule** across the guanidino group to release urea. *Formation of argininosuccinate* - This step involves the condensation of **citrulline** and **aspartate**, catalyzed by **argininosuccinate synthetase**. - It is an **ATP-dependent** reaction, not a hydrolysis. *Formation of citrulline* - Occurs when **carbamoyl phosphate** condenses with **ornithine**, catalyzed by **ornithine transcarbamylase**. - This reaction involves the removal of a phosphate group, not the addition of water. *Formation of ornithine* - **Ornithine** is a substrate for the formation of citrulline and is also regenerated at the end of the cycle from arginine. - Its formation from arginine is a **hydrolysis** reaction, but simply stating "formation of ornithine" is less specific than "cleavage of arginine," which directly describes the hydrolytic event.

Q: Lipogenesis is stimulated by?

Insulin. ***Insulin*** - **Insulin** is a key anabolic hormone that promotes the synthesis and storage of fat (lipogenesis) by increasing the uptake of glucose into adipose tissue and stimulating enzymes involved in fatty acid synthesis. - It enhances the conversion of excess carbohydrates into **triglycerides** for storage. *Glucagon* - **Glucagon** is a catabolic hormone that primarily promotes the breakdown of glycogen (glycogenolysis) and fat (lipolysis) to release glucose and fatty acids into the bloodstream, especially during fasting. - It generally **inhibits** lipogenesis and stimulates **gluconeogenesis**. *Epinephrine* - **Epinephrine** (adrenaline) is a stress hormone that promotes the breakdown of fat (lipolysis) to provide energy during acute stress or exercise. - It would **inhibit** lipogenesis, as its primary role is to mobilize energy stores. *Corticosteroids* - While **corticosteroids** can influence fat metabolism, their effect on lipogenesis is complex and often indirect. High levels can lead to fat redistribution (e.g., central obesity) rather than direct stimulation of overall lipogenesis. - Corticosteroids generally promote **lipolysis** in the extremities and can contribute to insulin resistance, which would hinder lipogenesis in some tissues.

Q: In which condition does serum appear milky white?

Increased Chylomicrons. ***Increased Chylomicrons*** - **Chylomicrons** are the largest lipoprotein particles (75-1200 nm) with the highest **triglyceride content (85-95%)**, giving serum a characteristic **milky white** or "creamy" appearance - This intense milky appearance occurs after **fatty meals** (postprandial lipemia) or in **Type I and V hyperlipidemias** (familial chylomicronemia syndrome) - The **light scattering** by these large particles makes the serum completely opaque, distinguishing it from other lipid abnormalities - Classic clinical finding: **"cream layer" forms on top** when lipemic serum stands overnight in refrigerator *Increased LDL* - Elevated **Low-Density Lipoprotein (LDL)** produces **clear to slightly hazy** serum, never milky white - LDL particles are much smaller (18-25 nm) than chylomicrons and contain primarily **cholesterol**, not triglycerides - High LDL is a cardiovascular risk factor but does not cause visible lipemia *Increased HDL* - **High-Density Lipoprotein (HDL)** elevation results in **clear serum** - HDL particles are the smallest (5-12 nm) and densest lipoproteins - High HDL is protective and causes no turbidity *Increased VLDL* - **Very Low-Density Lipoprotein (VLDL)** elevation can cause **turbid or hazy** serum in severe hypertriglyceridemia, but typically less intensely milky than chylomicrons - VLDL particles are smaller (30-80 nm) than chylomicrons with lower triglyceride content (50-65%) - In Type IV hyperlipidemia (isolated VLDL elevation), serum appears uniformly turbid without cream layer formation - The most dramatic "milky white" appearance is specifically associated with **chylomicronemia**

Q: Which amino acid is used by the liver in the urea cycle?

Aspartate. ***Aspartate*** - **Aspartate** provides the second nitrogen atom to the urea cycle, directly contributing to the formation of **argininosuccinate** through condensation with citrulline. - It is crucial for the efficient removal of **ammonia** in the form of urea. *Glutamine* - **Glutamine** transports ammonia from peripheral tissues to the liver and kidneys, but it is typically deamidated to **glutamate** before its nitrogen can enter the urea cycle. - While it's a major ammonia carrier, it's not directly incorporated into urea as an intact amino acid. *Glutamate* - **Glutamate** can donate its amino group to form **aspartate** (via transamination with oxaloacetate) or release ammonia directly (via glutamate dehydrogenase), both of which then enter the urea cycle. - However, glutamate itself is not directly incorporated into the urea molecule in the same way aspartate is. *Ornithine* - **Ornithine** is an amino acid that participates in the urea cycle as a carrier molecule, being regenerated at the end of each cycle. - While essential for the cycle to function, it is not "used" in the sense of being consumed or providing nitrogen for urea formation - rather it acts as a catalytic intermediate that is recycled.

Q: Which isoenzyme of lactate dehydrogenase (LDH) is predominantly elevated in liver injury?

LDH-5. ***LDH-5 isoenzyme most significant in hepatic conditions*** - **LDH-5** is the predominant isoenzyme found in the **liver** and skeletal muscle. - An elevation of **LDH-5** is highly indicative of **hepatocellular damage** or injury. *LDH-1 isoenzyme associated with cardiac tissue* - **LDH-1** is primarily present in the **heart** and red blood cells. - Its elevation suggests conditions like **myocardial infarction** or hemolytic anemia, not liver injury. *LDH-3 isoenzyme typical in respiratory system* - **LDH-3** is found in the **lungs**, kidneys, and other tissues. - While it can be elevated in **pulmonary embolism** or renal disease, it is not specific for liver injury. *LDH-2 isoenzyme linked to erythrocyte metabolism* - **LDH-2** is abundant in **red blood cells** and also found in the heart and kidneys. - Elevations are often seen in conditions involving **hemolysis** or myocardial damage, similar to LDH-1.

Q: Which of the following is a Fat Soluble vitamin?

Vitamin A. ***Vitamin A*** - **Vitamin A** is one of the four essential **fat-soluble vitamins** (A, D, E, K), meaning it dissolves in fat and is stored in the body's fatty tissues and liver. - Its **absorption** relies on dietary fats and bile salts, distinguishing it from water-soluble vitamins. *Thiamine* - **Thiamine** (Vitamin B1) is a **water-soluble vitamin** that plays a crucial role in metabolism, converting carbohydrates into energy. - It is **not stored** in large amounts in the body and needs to be replenished regularly through diet. *Niacin* - **Niacin** (Vitamin B3) is a **water-soluble vitamin** vital for cellular metabolism, involving in DNA repair and steroid hormone synthesis. - It is readily excreted in urine when consumed in excess and does not accumulate in fatty tissues. *Riboflavin* - **Riboflavin** (Vitamin B2) is a **water-soluble vitamin** essential for energy production, cellular function, and the metabolism of fats, drugs, and steroids. - Like other B vitamins, it has limited storage in the body and is not dissolved in fat.

Question 1

What is the most sensitive biochemical marker for a 7-day old myocardial infarction?

Accepted Answer

Troponin I/T

Answer

CPK MB

Answer

LDH

Answer

Myoglobin

Question 2

What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?

Accepted Answer

2 ATP, 2 NADH

Answer

4 ATP, 2 NADH

Answer

4 ATP, 4 NADH

Answer

2 ATP, 4 NADH

Question 3

Which enzyme is required for cutting the DNA strand during synthesis?

Accepted Answer

Topoisomerase

Answer

DNA polymerase

Answer

DNA ligase

Answer

Helicase

Question 4

Which of the following is not a component of a nucleotide?

Accepted Answer

Fatty acid

Answer

Sugar

Answer

Base

Answer

Phosphate

Question 5

Hydrolysis occurs at which step of urea cycle ?

Accepted Answer

Cleavage of arginine

Answer

Formation of ornithine

Answer

Formation of argininosuccinate

Answer

Formation of citrulline

Question 6

Lipogenesis is stimulated by?

Accepted Answer

Insulin

Answer

Glucagon

Answer

Epinephrine

Answer

Corticosteroids

Question 7

In which condition does serum appear milky white?

Accepted Answer

Increased Chylomicrons

Answer

Increased LDL

Answer

Increased HDL

Answer

Increased VLDL

Question 8

Which amino acid is used by the liver in the urea cycle?

Accepted Answer

Aspartate

Answer

Glutamine

Answer

Glutamate

Answer

Ornithine

Question 9

Which isoenzyme of lactate dehydrogenase (LDH) is predominantly elevated in liver injury?

Accepted Answer

LDH-5

Answer

LDH-3

Answer

LDH-1

Answer

LDH-2

Question 10

Which of the following is a Fat Soluble vitamin?

Accepted Answer

Vitamin A

Answer

Thiamine

Answer

Niacin

Answer

Riboflavin

NEET-PG 2012 — Biochemistry