NEET-PG 2012 Practice Questions

Q: Which of the following enzymes does not catalyze a reaction that directly produces ATP via substrate-level phosphorylation?

Hexokinase. ***Correct: Hexokinase*** **Hexokinase** catalyzes the transfer of a phosphate group from **ATP to glucose**, producing **glucose-6-phosphate** and ADP. This step **consumes ATP** rather than producing it via substrate-level phosphorylation. **Substrate-level phosphorylation** directly synthesizes ATP from ADP by transferring a high-energy phosphate group from a phosphorylated substrate; hexokinase performs the **opposite reaction** (ATP consumption). *Incorrect: Pyruvate kinase* **Pyruvate kinase** catalyzes the transfer of a phosphate group from **phosphoenolpyruvate (PEP)** to ADP, forming **pyruvate** and ATP. This is a classic example of **substrate-level phosphorylation** in glycolysis, directly generating ATP. *Incorrect: Succinate thiokinase* **Succinate thiokinase** (also known as succinyl-CoA synthetase) catalyzes the conversion of **succinyl-CoA to succinate**, simultaneously forming **GTP** (or ATP in some organisms) from GDP (or ADP) and inorganic phosphate. The GTP produced can be converted to ATP through nucleoside diphosphate kinase, representing substrate-level phosphorylation in the TCA cycle. *Incorrect: Phosphoglycerate kinase* **Phosphoglycerate kinase** catalyzes the transfer of a phosphate group from **1,3-bisphosphoglycerate** to ADP, yielding **3-phosphoglycerate** and ATP. This is a key enzymatic step in glycolysis that directly produces ATP through **substrate-level phosphorylation**.

Q: The rate-limiting step in glycolysis is catalyzed by?

Phosphofructokinase. ***Phosphofructokinase*** - **Phosphofructokinase-1 (PFK-1)** is the primary regulatory enzyme and **rate-limiting step** in glycolysis. - It catalyzes the irreversible phosphorylation of **fructose-6-phosphate to fructose-1,6-bisphosphate**, a crucial commitment step. *Enolase* - **Enolase** catalyzes the conversion of **2-phosphoglycerate to phosphoenolpyruvate** in glycolysis. - While essential for glycolysis, it is not the rate-limiting step. *Glucokinase* - **Glucokinase** catalyzes the phosphorylation of glucose to **glucose-6-phosphate** in the liver and pancreatic beta cells. - This is the first step in glycolysis but is not the rate-limiting step for the entire pathway once glucose has entered the cell. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of glycolysis, converting **phosphoenolpyruvate to pyruvate**. - Although it is a regulated enzyme, it is not the primary rate-limiting step that controls the overall flux through the glycolytic pathway.

Q: Which reaction requires Vitamin B1?

Oxidative decarboxylation. ***Oxidative decarboxylation*** - Vitamin B1, in its active form **thiamine pyrophosphate (TPP)**, is a crucial coenzyme for enzymes catalyzing **oxidative decarboxylation** reactions. - Key examples include the **pyruvate dehydrogenase complex** and **alpha-ketoglutarate dehydrogenase complex**, essential for cellular respiration and the citric acid cycle. *Transamination* - This type of reaction, involving the transfer of an **amino group**, primarily requires **pyridoxal phosphate (PLP)**, the active form of **Vitamin B6**. - It is vital for amino acid metabolism but does not utilize Vitamin B1. *Carboxylation* - **Carboxylation** reactions, which add a carboxyl group to a substrate, typically require **biotin** (Vitamin B7) as a coenzyme. - Examples include pyruvate carboxylase and acetyl-CoA carboxylase, which are not dependent on Vitamin B1. *None of the options* - As **oxidative decarboxylation** specifically requires Vitamin B1, this option is incorrect. - The other listed reactions depend on different vitamins as coenzymes.

Q: Cell-matrix adhesions are mediated by?

Integrins. ***Integrins*** - **Integrins** are transmembrane receptors that mediate cell adhesion to the **extracellular matrix (ECM)**, linking it to the cell's cytoskeleton. - They bind to various ECM components like **fibronectin**, **collagen**, and **laminin**. *Cadherins* - **Cadherins** are primarily involved in **cell-to-cell adhesion**, forming junctions like **adherens junctions** and **desmosomes**. - They are **calcium-dependent adhesion molecules** that do not directly bind to the extracellular matrix. *Selectins* - **Selectins** are cell adhesion molecules involved in **leukocyte rolling** and **adhesion to endothelial cells** during inflammation. - They mediate **transient cell-to-cell interactions**, not cell-matrix adhesion. *Calmodulin* - **Calmodulin** is a **calcium-binding protein** that acts as a signal transducer, regulating various intracellular processes. - It is involved in **calcium-dependent signaling pathways** and enzyme activation, not cell adhesion.

Q: In a measles outbreak, measles vaccine can be given to infants at what age range:

6-9 months. ***6-9 months*** - During a **measles outbreak**, infants as young as **6 months** can receive an early dose of the measles vaccine to provide protection. - This early vaccination is crucial in high-risk situations, even though the standard first dose is typically given at **12-15 months** of age. *2-3 months* - Administering the measles vaccine at **2-3 months** is generally avoided because of the presence of persistent **maternal antibodies**. - These antibodies can interfere with the infant's immune response to the vaccine, making it less effective. *3-5 months* - Similar to the 2-3 month range, **maternal antibodies** are still likely present and at sufficiently high levels in infants aged 3-5 months. - This interference reduces the vaccine's efficacy and the likelihood of developing a robust, lasting immune response. *2-7 months* - While this range includes the accepted 6-month age for early vaccination, infants under **6 months** still pose a challenge due to **maternal antibody interference**. - Combining these ages into a single range doesn't differentiate between the reduced efficacy in younger infants versus the permissible early vaccination at 6 months and older during an outbreak.

Q: What is the primary virulence factor of Neisseria gonorrhoeae?

Pili (fimbriae). ***Pili (fimbriae)*** - **Pili are the PRIMARY virulence factor** of *Neisseria gonorrhoeae*, essential for **initial attachment and colonization** of urogenital mucosa - Enable bacteria to **adhere to non-ciliated epithelial cells**, resisting mechanical clearance by urination and secretions - Undergo **antigenic variation** to evade host immune responses - Without pili, *N. gonorrhoeae* cannot establish infection *Endotoxin (lipooligosaccharide)* - *N. gonorrhoeae* possesses **LOS (lipooligosaccharide)** which causes inflammation and tissue damage - While important for pathogenesis, it is a **secondary virulence factor** - LOS contributes to symptoms but cannot cause infection without prior colonization via pili *Exotoxin* - *N. gonorrhoeae* does **not produce significant exotoxins** - Pathogenicity is mediated through **adherence factors (pili), LOS, and surface proteins** rather than secreted protein toxins - This is not a mechanism of gonococcal virulence *All of the above are incorrect* - This statement is false as **pili (fimbriae)** are definitively the primary virulence factor for *N. gonorrhoeae*

Q: Transitional cell carcinoma of the bladder is associated with which of the following?

None of the options. ***Schistosomiasis*** - Schistosomiasis, particularly from *Schistosoma haematobium*, is a well-known risk factor for **transitional cell carcinoma of the bladder** due to chronic irritation and inflammation [1]. - The association arises due to the **presence of eggs in the bladder**, leading to calcification and eventually cancer development. *Malaria* - Malaria is primarily associated with **hemolytic anemia** and does not have a direct correlation with **bladder cancer**. - Its causative agents, *Plasmodium* species, do not typically lead to **urological malignancies** like transitional cell carcinoma. *Ascarasis* - Ascarasis, caused by *Ascaris lumbricoides*, primarily affects the **intestines** and is more associated with gastrointestinal issues. - There is no significant link between ascarasis and the **development of bladder cancer**. *Any of d above* - As this option suggests all listed conditions, it incorrectly implies that **malaria** and **ascarasis** are linked to bladder cancer, which they are not. - Transitional cell carcinoma is specifically associated with **schistosomiasis**, making this option misleading. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lower Urinary Tract and Male Genital System, pp. 968-970.

Q: Half-life of iodine-131 is

8 days. ***8 days*** - The half-life of **iodine-131** is approximately 8.02 days, which makes it suitable for both diagnostic imaging and therapeutic applications in thyroid conditions. - This specific half-life allows sufficient time for the isotope to localize in the thyroid gland and deliver a therapeutic dose, while also ensuring it clears from the body relatively quickly to minimize long-term radiation exposure. *8 hours* - A half-life of 8 hours would be too short for many therapeutic applications of iodine-131, as it would decay too rapidly to deliver an effective dose to the thyroid. - Isotopes with such short half-lives are typically used for diagnostic imaging where rapid clearance and minimal patient exposure are paramount, such as **technetium-99m**. *8 weeks* - A half-life of 8 weeks would be excessively long for clinical use of iodine-131, leading to prolonged radiation exposure for the patient. - Such long half-lives increase the risk of adverse effects from cumulative radiation, making it unsuitable for routine diagnostic or therapeutic procedures. *8 months* - A half-life of 8 months is impractically long for any medical application requiring regular administration, as it would lead to very high and persistent radiation doses. - This duration would result in significant and unacceptable long-term radiation hazards, making its use unfeasible for imaging or therapy.

Question 1

Which of the following enzymes does not catalyze a reaction that directly produces ATP via substrate-level phosphorylation?

Accepted Answer

Hexokinase

Answer

Pyruvate kinase

Answer

Succinate thiokinase

Answer

Phosphoglycerate kinase

Question 2

The rate-limiting step in glycolysis is catalyzed by?

Accepted Answer

Phosphofructokinase

Answer

Enolase

Answer

Glucokinase

Answer

Pyruvate kinase

Question 3

Which reaction requires Vitamin B1?

Accepted Answer

Oxidative decarboxylation

Answer

None of the options

Answer

Carboxylation

Answer

Transamination

Question 4

Cell-matrix adhesions are mediated by?

Accepted Answer

Integrins

Answer

Selectins

Answer

Calmodulin

Answer

Cadherins

Question 5

Which of the following represents the most significant regulatory control point among these TCA cycle reactions?

Accepted Answer

Isocitrate to Alpha-ketoglutarate (Isocitrate dehydrogenase)

Answer

Succinyl-CoA to Succinate (Succinyl-CoA synthetase)

Answer

Acetyl-CoA + Oxaloacetate to Citrate (Citrate synthase)

Answer

Alpha-ketoglutarate to Succinyl-CoA (Alpha-ketoglutarate dehydrogenase complex)

Question 6

In a measles outbreak, measles vaccine can be given to infants at what age range:

Accepted Answer

6-9 months

Answer

2-3 months

Answer

3-5 months

Answer

2-7 months

Question 7

What is the primary virulence factor of Neisseria gonorrhoeae?

Accepted Answer

Pili (fimbriae)

Answer

All of the above are incorrect

Answer

Endotoxin (lipooligosaccharide)

Answer

Exotoxin

Question 8

Transitional cell carcinoma of the bladder is associated with which of the following?

Accepted Answer

None of the options

Answer

Malaria

Answer

Schistosomiasis

Answer

Ascariasis

Question 9

What is the primary factor that determines the resting membrane potential in a nerve fiber?

Accepted Answer

Is primarily determined by the equilibrium potential of potassium ions.

Answer

Is equal to the resting potential of cardiac muscle fibers.

Answer

Can be accurately measured using intracellular electrodes.

Answer

Increases with elevated extracellular potassium concentration.

Question 10

Half-life of iodine-131 is

Accepted Answer

8 days

Answer

8 hours

Answer

8 weeks

Answer

8 months

NEET-PG 2012

Biochemistry

Community Medicine

Microbiology

Pathology

Physiology

Radiology