INI-CET 2025 - Biochemistry PYQs Practice Questions

Q: A child presents with fatigue and hepatomegaly. Liver enzymes (ALT, AST) are elevated. Ketosis was significant. Liver biopsy shows excess glycogen accumulation. After feeding, blood glucose levels rise, but there is no rise in glucose after overnight fasting. Which of the following enzyme deficiencies is most likely responsible for this presentation?

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - This deficiency is characteristic of Glycogen Storage Disease type I (**GSD I** or **Von Gierke disease**), which prevents the final step of both glycogenolysis and gluconeogenesis (conversion of **Glucose-6-Phosphate** to free glucose). - The inability to release free glucose from the liver, especially during fasting, causes severe **fasting hypoglycemia**, significant **ketosis**, and massive **hepatomegaly** due to trapped glycogen and fat accumulation. *Liver phosphorylase* - Deficiency in **Liver phosphorylase** (GSD VI or Hers disease) impairs the breakdown of glycogen but does not affect the gluconeogenesis pathway. - While it leads to hepatomegaly and hypoglycemia, the symptoms are generally **milder** than GSD I, and the profound metabolic derangements (severe ketosis, lactic acidosis) seen in this case are typically less pronounced. *Muscle phosphorylase* - This enzyme deficiency, known as **GSD V** or **McArdle disease**, primarily affects the skeletal muscle, leading to muscle pain, cramps, and **exercise intolerance**. - It does not cause hepatomegaly or issues with hepatic glucose release and therefore is an **unlikely** cause of fasting hypoglycemia in a child. *Phosphofructokinase* - Phosphofructokinase (GSD VII or **Tarui disease**) deficiency affects both muscle and erythrocytes, presenting similar to McArdle disease with **exercise intolerance** and often mild hemolysis. - PFK deficiency primarily affects glycolysis and does not directly impair the liver's ability to release glucose via **gluconeogenesis** or the final G6Pase step during fasting. [Practice more Biochemistry PYQs on OnCourse]

Q: Cancer cells take up excess glucose because?

Lactate is produced even in the presence of Oxygen. ***Lactate is produced even in the presence of Oxygen*** - This phenomenon is known as the **Warburg effect** (or aerobic glycolysis). Cancer cells preferentially ferment glucose to lactate, even when adequate **oxygen** is available, bypassing efficient oxidative phosphorylation. - This inefficient use of glucose allows rapid generation of metabolic intermediates (e.g., carbon backbones) required for the synthesis of **lipids**, proteins, and nucleic acids needed for rapid cell proliferation. *High NADH/NAD ratio* - A **high NADH/NAD+ ratio** signals abundance of reducing equivalents, which would typically inhibit glycolysis and favor oxidative phosphorylation. - Rapid glycolysis, as seen in cancer (Warburg effect), requires the constant regeneration of **NAD+** from NADH via lactate dehydrogenase for the pathway to continue. *High GLUT2* - While cancer cells increase glucose uptake by overexpressing glucose transporters, the typically overexpressed transporter in many solid tumors is **GLUT1**, not GLUT2. - **GLUT2** is primarily found in the liver, kidney, and pancreatic beta cells and is less commonly the primary high-affinity transporter driving the intense uptake seen in malignant cells. *Absence of Oxygen* - The characteristic metabolic change in cancer is that glucose uptake and lactate production occur despite the **presence of oxygen** (aerobic glycolysis). - If oxygen were truly absent (anaerobic conditions), all cell types would produce lactate; hence, the defining feature of cancer is the metabolic shift occurring in an **oxygenated environment**. [Practice more Biochemistry PYQs on OnCourse]

Q: How many PCR cycles are needed to produce half the amplified DNA copies produced by 27 cycles of PCR?

26. ***26 (Correct)*** - PCR amplification follows an **exponential growth** pattern, where the template DNA is theoretically doubled in each cycle. The product quantity is proportional to 2^n (where n is the cycle number). - If 27 cycles produce 2^27 copies, half of this amount is calculated as 2^27 / 2, which mathematically simplifies to 2^(27-1). Therefore, exactly **26 cycles** are needed. *25 (Incorrect)* - 25 cycles would produce 2^25 copies. Since 2^27 = 4 × 2^25, 25 cycles yield only **one-quarter** (25%) of the DNA produced by 27 cycles, not half. - In exponential amplification, a difference of two cycles corresponds to a **four-fold change** in product amount. *13 (Incorrect)* - 13 cycles would produce 2^13 copies, which represents an extremely small fraction of the total DNA generated after 27 cycles (2^27). - This amount belongs to the early phase of the reaction and is mathematically too low to be relevant to the calculation for half the **final product quantity**. *12 (Incorrect)* - 12 cycles produce 2^12 copies, which is 2^15 times less than 2^27 copies. - This number is irrelevant as it doesn't align with the required calculation based on the principles of **geometric progression** in PCR. [Practice more Biochemistry PYQs on OnCourse]

Q: Mechanism of inhibition caused by cyanide and carbon monoxide poisoning involves inhibition of which enzyme?

Cytochrome C oxidase. ***Cytochrome C oxidase*** - Cyanide and carbon monoxide are **powerful inhibitors of Cytochrome C oxidase (Complex IV)** in the electron transport chain. - Cyanide binds to the **ferric iron (Fe³⁺)** in Complex IV, while carbon monoxide also binds to Complex IV, preventing oxygen from binding. - Inhibition of Complex IV **stops the transfer of electrons to oxygen**, halting the entire process of oxidative phosphorylation and cellular respiration, leading to **cellular hypoxia and energy deficit**. *Incorrect: NADH dehydrogenase* - This enzyme, also known as Complex I, is primarily inhibited by compounds like **Rotenone** and **Amytal**. - While crucial for the ETC, it is not the target of carbon monoxide or cyanide. *Incorrect: Succinate dehydrogenase* - This enzyme, known as Complex II, is an integral part of both the ETC and the Krebs cycle. - It is specifically inhibited by compounds like **Malonate** and is not the primary target in cyanide or carbon monoxide poisoning. *Incorrect: Cytochrome C oxidoreductase* - This enzyme represents Complex III (also called the Cytochrome bc1 complex). - It transfers electrons from ubiquinone to cytochrome C, but its inhibition is not the primary mechanism of action for cyanide or carbon monoxide, which directly target Complex IV. [Practice more Biochemistry PYQs on OnCourse]

Question 1

A child presents with fatigue and hepatomegaly. Liver enzymes (ALT, AST) are elevated. Ketosis was significant. Liver biopsy shows excess glycogen accumulation. After feeding, blood glucose levels rise, but there is no rise in glucose after overnight fasting. Which of the following enzyme deficiencies is most likely responsible for this presentation?

Accepted Answer

Glucose-6-phosphatase

Answer

Liver phosphorylase

Answer

Muscle phosphorylase

Answer

Phosphofructokinase

Question 2

Cancer cells take up excess glucose because?

Accepted Answer

Lactate is produced even in the presence of Oxygen

Answer

High NADH/NAD ratio

Answer

High GLUT2

Answer

Absence of Oxygen

Question 3

How many PCR cycles are needed to produce half the amplified DNA copies produced by 27 cycles of PCR?

Accepted Answer

26

Answer

12

Answer

25

Answer

13

Question 4

Mechanism of inhibition caused by cyanide and carbon monoxide poisoning involves inhibition of which enzyme?

Accepted Answer

Cytochrome C oxidase

Answer

Succinate dehydrogenase

Answer

NADH dehydrogenase

Answer

Cytochrome C oxidoreductase

Question 5

A deficiency of Glucose-6-Phosphatase is associated with which of the following bilirubin patterns?

Accepted Answer

Direct bilirubin normal, indirect bilirubin increased, and urobilinogen normal

Answer

Indirect bilirubin increased, direct bilirubin normal, and raised urine urobilinogen

Answer

Direct bilirubin increased, indirect bilirubin decreased, and urine urobilinogen/urobilin increased

Answer

Raised direct bilirubin, normal indirect bilirubin, and absent urine bilirubin

Question 6

What is the pH of the zwitterion as shown in the titration curve of glycine?

Accepted Answer

6

Answer

2

Answer

9

Answer

4.4

Question 7

The PRIMARY mechanism of toxicity of which substance is NOT through Cytochrome C oxidase inhibition?

Accepted Answer

Carbon monoxide (CO)

Answer

Nitric oxide (NO)

Answer

Cyanide (CN)

Answer

Oligomycin

Question 8

A 90 kg obese man is taking a carbohydrate-rich diet. Which of the following enzymes/intermediates will be elevated in him? 1. Malonyl CoA 2. Acetyl CoA Carboxylase 3. PDH 4. Citrate Lyase

Accepted Answer

$1,2,3,4$

Answer

$1,2,3$

Answer

$1,3,4$

Answer

$2,3,4$

Question 9

Match the following blotting techniques with the type of biomolecule they detect: Blotting Technique Detects A. Southern blot 1. RNA B. Northern blot 2. Lipids C. Western blot 3. DNA D. Eastern blot 4. Protein

Accepted Answer

A-3, B-1, C-4, D-2

Answer

A-4, B-2, C-1, D-3

Answer

A-2, B-4, C-3, D-1

Answer

A-1, B-3, C-2, D-4

Question 10

IGF-1 and IGF-2 are structurally most similar to which of the following molecules?

Accepted Answer

Insulin

Answer

Preproinsulin

Answer

Proinsulin

Answer

C-peptide

INI-CET 2025 — Biochemistry