NEET-PG 2013

1544 Questions — Page 23 of 155

Biochemistry

6 questions

Q221

What is a physiological uncoupler?

Q222

Which of the following is a natural uncoupler found in brown adipose tissue?

Q223

ATP is generated in the Electron Transport Chain (ETC) specifically by which enzyme?

Q224

Which is the primary energy molecule that gives approximately 7.3 kcal/mol?

Q225

Which of the following enzymes is classified as a serine protease?

Q226

According to IUB system, hydrolases belong to which class?

NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs

Question 221: What is a physiological uncoupler?

A. Thyroxine
B. Free fatty acids
C. Thermogenin (Correct Answer)
D. All of the options

Explanation: ***Correct: Thermogenin*** - **Thermogenin (uncoupling protein 1, UCP1)** is the primary physiological uncoupler found in brown adipose tissue - It directly facilitates the **leak of protons** back into the mitochondrial matrix, bypassing ATP synthase - This dissipates the **proton-motive force as heat** rather than producing ATP, making it the classic example of non-shivering thermogenesis - Essential for **temperature regulation** in neonates and cold adaptation in adults *Incorrect: Free fatty acids* - While free fatty acids can activate UCP1 and act as weak protonophores in some contexts, they are primarily **substrates for β-oxidation** and **activators** of thermogenin - They are not considered the primary physiological uncoupler, though they support uncoupling activity *Incorrect: Thyroxine* - **Thyroid hormone** increases metabolic rate and can upregulate the **expression of uncoupling proteins** - However, it does **not directly uncouple** oxidative phosphorylation - It acts as a metabolic regulator rather than a true uncoupler *Incorrect: All of the options* - Only thermogenin is the true physiological uncoupler by definition - The other substances play supportive or regulatory roles but are not direct uncouplers

Question 222: Which of the following is a natural uncoupler found in brown adipose tissue?

A. Thermogenin (Correct Answer)
B. 2,4-Nitrophenol
C. 2,4-Dinitrophenol
D. Oligomycin

Explanation: ***Correct: Thermogenin*** - Also known as **uncoupling protein 1 (UCP1)**, it is a **mitochondrial inner membrane protein** naturally expressed in **brown adipose tissue** - Thermogenin creates a **proton leak** across the inner mitochondrial membrane, bypassing ATP synthase and dissipating the proton gradient as heat, thereby mediating **non-shivering thermogenesis** - This is the only natural uncoupler among the options listed *Incorrect: 2,4-Nitrophenol* - This compound is **not a naturally occurring uncoupler** in mammalian tissues - While it can act as a synthetic uncoupler in laboratory settings, it is not found in biological systems *Incorrect: 2,4-Dinitrophenol* - This is a well-known **synthetic chemical uncoupler** of oxidative phosphorylation, historically used as a weight-loss drug (now banned due to toxicity) - It works by carrying protons across the inner mitochondrial membrane, but it is **not a natural biological molecule** found in the body *Incorrect: Oligomycin* - Oligomycin is an **inhibitor of ATP synthase (Complex V)**, not an uncoupler - It binds to the F0 subunit of ATP synthase, blocking the flow of protons through the enzyme and thereby preventing ATP synthesis - This blocks both the proton gradient dissipation AND ATP production, which is mechanistically different from uncoupling

Question 223: ATP is generated in the Electron Transport Chain (ETC) specifically by which enzyme?

A. Cl- ATPase
B. ADP Kinase
C. FoF1 ATPase (Correct Answer)
D. Na+/K+ ATPase

Explanation: ***FoF1 ATPase*** - The **FoF1 ATPase**, also known as **ATP synthase**, is the complex enzyme responsible for synthesizing ATP using the **proton gradient** generated by the electron transport chain. - The **Fo subunit** forms a channel that allows protons to flow back into the mitochondrial matrix, driving the rotation of the **F1 subunit** which catalyzes ATP synthesis from ADP and inorganic phosphate. *Na+/K+ ATPase* - This enzyme is a **pump** that actively transports **three sodium ions out** of the cell and **two potassium ions into** the cell, maintaining membrane potential. - It uses **ATP hydrolysis** as its energy source, meaning it **consumes ATP** rather than producing it directly in the ETC. *Cl- ATPase* - **Cl- ATPase** refers to a family of pumps that transport **chloride ions**, typically using ATP hydrolysis as an energy source. - These enzymes are involved in ion homeostasis and fluid balance, but they do **not generate ATP** in the electron transport chain. *ADP Kinase* - **ADP Kinase** is a general term for enzymes that catalyze the phosphorylation of ADP to ATP, often by transferring a phosphate group from another high-energy molecule. - While it produces ATP, it is not the specific enzyme that directly harnesses the **proton gradient** in the electron transport chain for oxidative phosphorylation.

Question 224: Which is the primary energy molecule that gives approximately 7.3 kcal/mol?

A. ATP (Correct Answer)
B. GTP
C. Glucose-6-phosphate
D. Creatine phosphate

Explanation: ***ATP*** - **Adenosine triphosphate (ATP)** is the primary energy currency of the cell, providing approximately **7.3 kcal/mol** upon hydrolysis of its terminal phosphate group. - This energy is released when ATP is converted to **ADP (adenosine diphosphate)** and an inorganic phosphate (Pi), driving various cellular processes. *GTP* - **Guanosine triphosphate (GTP)** is another nucleotide triphosphate that carries energy, but it is primarily involved in specific processes like **protein synthesis** and **signal transduction**, not as the ubiquitous primary energy molecule like ATP. - While it also releases energy upon hydrolysis, its standard free energy change is similar to ATP but it's not the main universal energy carrier. *Glucose-6-phosphate* - **Glucose-6-phosphate** is an important intermediate in **glycolysis** and **gluconeogenesis**, but it is not an energy-storing molecule in the same way as ATP. - Its high-energy phosphate bond is used in metabolic pathways, but it doesn't directly release 7.3 kcal/mol as a direct energy source for cellular work. *Creatine phosphate* - **Creatine phosphate** serves as an energy reserve in muscle and nerve cells, rapidly generating ATP from ADP during periods of intense activity. - While it is a high-energy phosphate compound, it functions to **replenish ATP** rather than being the direct energy molecule that performs cellular work.

Question 225: Which of the following enzymes is classified as a serine protease?

A. Pepsin
B. Trypsin (Correct Answer)
C. Carboxypeptidase
D. None of the options

Explanation: ***Trypsin*** - **Trypsin** is a digestive enzyme belonging to the **serine protease** family, characterized by a crucial **serine residue** in its active site. - It plays a vital role in protein digestion in the small intestine, cleaving peptide bonds on the carboxyl side of **lysine** or **arginine** residues. *Pepsin* - **Pepsin** is an aspartic protease, meaning it utilizes an **aspartate residue** in its active site for catalysis. - It primarily functions in the stomach, digesting proteins into smaller peptides in an **acidic environment**. *Carboxypeptidase* - **Carboxypeptidase** is a **metalloexopeptidase** that contains a zinc ion in its active site. - It removes amino acids one by one from the **carboxyl-terminal** end of polypeptide chains. *None of the options* - This option is incorrect because **trypsin** is indeed a well-known example of a serine protease.

Question 226: According to IUB system, hydrolases belong to which class?

A. EC-1
B. EC-2
C. EC-3 (Correct Answer)
D. EC-4

Explanation: ***EC-3*** - **Hydrolases** catalyze the **hydrolysis** of chemical bonds, which involves the addition of water to break the bond. - This class includes enzymes like **esterases**, **peptidases**, and **glycosidases**, all of which use water to cleave molecules. *EC-1* - **EC-1** refers to **oxidoreductases**, which catalyze **oxidation-reduction reactions**. - These enzymes are involved in the transfer of electrons or hydrogen atoms, not the hydrolysis of bonds. *EC-2* - **EC-2** represents **transferases**, enzymes that catalyze the **transfer of a functional group** from one molecule to another. - Examples include **kinases** and **transaminases**, which are distinct from hydrolytic enzymes. *EC-4* - **EC-4** encompasses **lyases**, which catalyze the **cleavage of various bonds** by means other than hydrolysis or oxidation, often forming double bonds. - This class includes enzymes like **decarboxylases** and **aldolases**, which are not primarily involved in breaking bonds with water.

Internal Medicine

1 questions

Q221

Which of the following is a non- modifiable risk factor for CHD -

Pathology

3 questions

Q221

Which of the following changes is NOT seen in atherosclerotic plaque at the time of rupture?

Q222

Which of the following cell types is classified as a labile cell?

Q223

Dystrophic calcification is seen in

NEET-PG 2013 - Pathology NEET-PG Practice Questions and MCQs

Question 221: Which of the following changes is NOT seen in atherosclerotic plaque at the time of rupture?

A. Inflammatory cell infiltration
B. Thick fibrous cap (Correct Answer)
C. Cell debris
D. Smooth muscle cell atrophy

Explanation: ***Smooth muscle cell hypertrophy*** - **Smooth muscle cell hypertrophy** is generally associated with stable plaques and does not typically occur in ruptured atherosclerotic plaques [2]. - At rupture, there is **loss of smooth muscle cells** and thinning of the fibrous cap, leading to plaque instability [2]. *Thin fibrosis cap* - A **thin fibrous cap** is a critical feature of vulnerable plaques, making them prone to rupture [2]. - It indicates a **weakened structure** that can no longer withstand the pressure of the underlying lipid core [2]. *Cell debris* - **Cell debris** is often found at the site of rupture, resulting from the necrosis of foam cells and smooth muscle cells. - This indicates **plaque instability** and contributes to the thrombus formation at the rupture site. *Multiple foam cap* - The presence of **multiple foam cells** reflectsing lipid accumulation in the plaque but does not contribute to the phenomenon of plaque rupture directly. - While foam cells are associated with rupture, a **foam cap** is not a recognized pathological finding at the time of rupture. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Cardiovascular Disease, pp. 271-272. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Cardiovascular Disease, pp. 268-270.

Question 222: Which of the following cell types is classified as a labile cell?

A. Liver parenchymal cells
B. Vascular smooth muscle cells
C. Surface epithelium (Correct Answer)
D. Neurons

Explanation: ***Surface epithelium*** - Surface epithelium is classified as **labile tissue**, meaning it undergoes constant regeneration due to its high turnover rate [1]. - Cells in this tissue are typically found in areas that experience frequent damage or abrasion, such as the skin and lining of the intestines. *Cardiac cell* - Cardiac cells are considered **permanent cells**, as they do not undergo significant regeneration after injury or damage. - Damage to cardiac cells typically leads to **fibrosis** rather than repair of the original tissue. *Liver parenchymal cell* - Liver parenchymal cells are categorized as **stable cells**, which can regenerate but do so under specific circumstances, such as injury. - They have a slower turnover rate compared to labile cells and do not constantly renew under normal conditions. *Vascular endothelial cells* - Vascular endothelial cells are considered **stable cells** as well, typically maintaining a stable population but capable of regeneration following injury. - They do not have the same rapid turnover and regeneration capability as labile cells do, especially under normal physiological conditions. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 113-115.

Question 223: Dystrophic calcification is seen in

A. Vitamin A intoxication
B. Atheromatous plaque (Correct Answer)
C. Milk alkali syndrome
D. Hyperparathyroidism

Explanation: ***Atheromatous plaque*** - Dystrophic calcification occurs in **local areas of tissue injury**, like atheromatous plaques, where necrotic debris provides a nidus for calcification [1]. - It's commonly observed in chronic **atherosclerosis**, leading to the deposition of calcium in the damaged arterial walls [1]. *Hyperparathyroidism* - Typically associated with **metastatic calcification** due to elevated calcium levels, not dystrophic calcification [2][3]. - It results in renal, pulmonary, or vascular calcifications rather than calcifications in previously damaged tissues [3]. *Milk alkali syndrome* - Involves **hypercalcemia** and can lead to calcifications, but they are primarily **metastatic** rather than dystrophic [2][3]. - The syndrome results from excess calcium intake and is associated with renal injury rather than tissue necrosis. *Vitamin A intoxication* - Can cause **hyperostosis** and **calcifications**, but these are diffuse and not primarily dystrophic in nature. - The calcifications in this condition do not stem from necrotic tissue but rather are due to toxicity effects on bone metabolism. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 506-507. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 134-135. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 76-77.