NEET-PG 2018 - Biochemistry Practice Questions

Q: Elevated levels of vanillylmandelic acid (VMA) in urine are characteristically found in which of the following conditions?

Pheochromocytoma. ***Pheochromocytoma*** - **Vanillylmandelic acid (VMA)** is a major urinary metabolite of the **catecholamines epinephrine and norepinephrine**. - **Pheochromocytoma** is a tumor of the adrenal medulla that secretes excessive amounts of these catecholamines, leading to significantly elevated VMA levels in urine. *Phenyl ketonuria* - Characterized by the inability to metabolize **phenylalanine** due to a deficiency of the enzyme **phenylalanine hydroxylase**. - Leads to accumulation of **phenylalanine** and its metabolites, such as **phenylpyruvic acid**, not VMA. *Diabetic ketoacidosis* - A severe complication of **diabetes mellitus** resulting from a profound insulin deficiency, leading to high blood glucose and **ketone body** production. - While it alters metabolism, it does not directly lead to elevated VMA levels. *Alkaptonuria* - A rare genetic disorder caused by a deficiency of the enzyme **homogentisate 1,2-dioxygenase**, involved in **tyrosine metabolism**. - Results in the accumulation of **homogentisic acid**, which is excreted in the urine and turns dark on exposure to air, but does not involve VMA.

Q: In Cystinuria, all of the following amino acids are excreted in urine, except:-

Leucine. ***Leucine*** - Cystinuria is a disorder characterized by impaired transport of **dibasic amino acids** and **cystine**, not neutral amino acids like leucine. - Therefore, **leucine** would be properly reabsorbed and not significantly excreted in the urine. *Cystine* - **Cystine** is one of the four amino acids whose reabsorption is impaired in cystinuria, leading to its excessive excretion in urine and potential **kidney stone** formation. - The defect is in the **renal tubular transport system** for dibasic amino acids and cystine. *Ornithine* - **Ornithine** is a **dibasic amino acid** and its renal reabsorption is defective in cystinuria. - Like cystine, ornithine is excessively excreted in the urine due to the shared transport system. *Arginine* - **Arginine** is also a **dibasic amino acid** whose renal reabsorption is impaired in cystinuria. - Its presence in the urine is increased, along with cystine, ornithine, and lysine, forming the classic pattern of amino acid excretion in this condition.

Q: Which of the following indicates the 'flipping effect'?

LDH1 > LDH2. ***LDH1 > LDH2*** - The "flipping effect" specifically refers to an **increase in LDH1 activity** such that it surpasses LDH2 activity. - This pattern is a classic indicator of **myocardial injury**, particularly a **myocardial infarction (heart attack)**. *LDH2 > LDH1* - This is the **normal pattern** of LDH isoenzyme distribution in healthy individuals. - In healthy serum, **LDH2 is typically higher than LDH1**. *LDH2 > LDH3* - While LDH2 is normally higher than LDH3, this comparison is **not characteristic** of the "flipping effect." - The "flipping effect" specifically involves the relationship between **LDH1 and LDH2**. *LDH2 > LDH4* - This relationship is generally true in both normal and abnormal conditions, as **LDH2 is typically abundant** in serum. - It does not represent the specific **diagnostic pattern** known as the "flipping effect."

Q: Most abundant form of pro-vitamin A is:-

Beta carotene. ***Beta carotene*** - **Beta-carotene** is the most widely distributed and abundant form of **provitamin A** in plants. - It yields two molecules of **retinal** upon enzymatic cleavage, which can then be converted to **retinol**. *Retinol* - **Retinol** is the active form of **vitamin A** and is not a pro-vitamin. - It is derived from pro-vitamin A carotenoids or ingested directly as preformed vitamin A. *Alpha-carotene* - **Alpha-carotene** is another form of **provitamin A**, but it is **less abundant** than beta-carotene. - It yields only one molecule of **retinal** because of its asymmetric structure. *Retinaldehyde* - **Retinaldehyde** (retinal) is an intermediate metabolite in the **vitamin A pathway**, converted from provitamin A or retinol. - It is not considered a pro-vitamin but rather a direct precursor to other active forms of vitamin A, such as **retinol** and **retinoic acid**.

Q: Active metabolite in the synthesis of fatty acids is:

Malonyl CoA. ***Malonyl CoA*** - **Malonyl CoA** is the immediate **two-carbon donor** in fatty acid synthesis, formed from acetyl CoA and bicarbonate. - It adds **two-carbon units** to the growing fatty acid chain during each cycle of synthesis, making it the primary active metabolic form in this process. *Stearate (an end product of fatty acid synthesis)* - **Stearate** is a **saturated fatty acid end product** (C18:0) of fatty acid synthesis, not an active metabolite that directly participates in the elongation process. - While it is a result of fatty acid synthesis, it does not serve as a building block for further elongation in the manner of malonyl CoA. *Acetyl CoA (a precursor in fatty acid synthesis)* - **Acetyl CoA** is the **initial precursor** for fatty acid synthesis, which is then carboxylated to form malonyl CoA. - It is not the *active* two-carbon donor during the elongation steps of fatty acid synthesis itself, but rather the substrate for malonyl CoA synthesis. *Palmitate (an end product of fatty acid synthesis)* - **Palmitate** is the **primary 16-carbon saturated fatty acid** and is the usual end product of *de novo* fatty acid synthesis in humans. - Like stearate, it is an end product and does not serve as an active metabolic intermediate for chain elongation during the synthesis process itself.

Q: Which protein plays a role in the process shown below? (Recent NEET Pattern 2018)

Clathrin. ***Clathrin*** - **Clathrin** is the primary protein involved in forming coated pits and vesicles during receptor-mediated endocytosis, which is the process shown in the image. - It forms a characteristic **triskelion structure** (three-legged complex) that assembles into a polyhedral lattice around the budding vesicle. - This protein coat provides the mechanical force needed to bend the membrane and form vesicles for internalizing substances. - Clathrin-mediated endocytosis is the **most well-characterized pathway** for receptor-mediated uptake of molecules. *Pallidin* - Pallidin is a component of the **BLOC-1 complex** (Biogenesis of Lysosome-related Organelles Complex-1). - It is involved in the biogenesis and trafficking of lysosome-related organelles, not in the primary vesicle formation during endocytosis. - Mutations in pallidin cause Hermansky-Pudlak syndrome type 9. *Ubiquitin* - Ubiquitin is a small regulatory protein that marks other proteins for degradation via the **proteasome** or targets them for endocytosis. - While ubiquitination can signal for receptor internalization, ubiquitin itself does not form vesicle coats or directly participate in vesicle budding. *Actin* - Actin is a cytoskeletal protein that plays a supporting role in endocytosis, particularly in membrane invagination and vesicle scission. - It provides mechanical support and force for vesicle movement but is **not the primary coat protein** forming the vesicle structure during initial budding.

Q: The image shows the world's fastest athlete. Which of the following is used by his muscles for meeting energy demands to create world records in seconds?

Phosphocreatine. ***Phosphocreatine*** - **Phosphocreatine** is a high-energy phosphate compound stored in muscle cells, providing the most rapid source of ATP regeneration for short, intense bursts of activity lasting seconds. - During explosive activities like sprinting (0-10 seconds), the enzyme **creatine kinase** rapidly transfers a phosphate group from phosphocreatine to ADP, re-synthesizing ATP almost instantaneously. - This **phosphagen system** (ATP-PC system) is the primary energy source for world-record sprints, allowing for maximal power output before glycolysis can ramp up. *Phosphofructokinase* - **Phosphofructokinase (PFK)** is a key regulatory enzyme in glycolysis, not an energy substrate itself. - While glycolysis provides ATP for sustained high-intensity exercise (10 seconds to 2 minutes), it is significantly slower than the phosphocreatine system. - PFK catalyzes the rate-limiting step of glycolysis but does not directly provide the immediate energy for explosive movements in seconds. *Glucose 1-phosphate* - **Glucose 1-phosphate** is an intermediate in glycogenolysis (glycogen breakdown) and glycogen synthesis. - It must be converted to glucose 6-phosphate and then proceed through glycolysis to generate ATP, which takes longer than direct phosphocreatine utilization. - This pathway supports energy production but is not the immediate source for explosive power in seconds. *Creatine phosphokinase* - **Creatine phosphokinase (CPK)** or **creatine kinase (CK)** is the enzyme that catalyzes the transfer of phosphate from phosphocreatine to ADP. - While essential for the process, it is the enzyme facilitator, not the energy substrate itself. - The question asks what is "used" for energy, which refers to the substrate (phosphocreatine), not the enzyme.

Question 1

Elevated levels of vanillylmandelic acid (VMA) in urine are characteristically found in which of the following conditions?

Accepted Answer

Pheochromocytoma

Answer

Phenyl ketonuria

Answer

Diabetic ketoacidosis

Answer

Alkaptonuria

Question 2

In Cystinuria, all of the following amino acids are excreted in urine, except:-

Accepted Answer

Leucine

Answer

Cystine

Answer

Ornithine

Answer

Arginine

Question 3

Which of the following indicates the 'flipping effect'?

Accepted Answer

LDH1 > LDH2

Answer

LDH2 > LDH1

Answer

LDH2 > LDH3

Answer

LDH2 > LDH4

Question 4

Most abundant form of pro-vitamin A is:-

Accepted Answer

Beta carotene

Answer

Retinol

Answer

Alpha-carotene

Answer

Retinaldehyde

Question 5

Active metabolite in the synthesis of fatty acids is:

Accepted Answer

Malonyl CoA

Answer

Stearate

Answer

Acetyl CoA

Answer

Palmitate

Question 6

Which protein plays a role in the process shown below? (Recent NEET Pattern 2018)

Accepted Answer

Clathrin

Answer

Ubiquitin

Answer

Pallidin

Answer

Actin

Question 7

What is correct about the vitamin shown below?

Accepted Answer

γ-carboxylation of glutamic acid residues to clotting factor 2,7, 9 and 10

Answer

γ-carboxylation of glycine residues to clotting factor 2,7, 9 and 10

Answer

γ-carboxylation of glutathione residues to clotting factor 2,7, 9 and 10

Answer

γ-carboxylation of aspartic acid residues to clotting factor 2,7, 9 and 10

Question 8

The image shows the world's fastest athlete. Which of the following is used by his muscles for meeting energy demands to create world records in seconds?

Accepted Answer

Phosphocreatine

Answer

Phosphofructokinase

Answer

Glucose 1-phosphate

Answer

Creatine phosphokinase

NEET-PG 2018 — Biochemistry