NEET-PG 2012 Practice Questions

Q: Which enzyme primarily initiates the electron transport process in oxidative phosphorylation?

NADH dehydrogenase. ***Correct NADH dehydrogenase*** - **NADH dehydrogenase**, also known as Complex I, is the enzyme that accepts electrons from **NADH** during oxidative phosphorylation, initiating the electron transport chain. - This enzyme **oxidizes NADH** to NAD+ and pumps protons from the mitochondrial matrix to the intermembrane space, contributing to the **proton gradient**. *Incorrect Pyruvate kinase* - **Pyruvate kinase** is an enzyme involved in **glycolysis**, catalyzing the final step of converting phosphoenolpyruvate to pyruvate. - It functions in the **cytoplasm** and is not directly involved in the electron transport chain or oxidative phosphorylation. *Incorrect Succinyl CoA thiokinase* - **Succinyl CoA thiokinase** (also known as succinate thiokinase or succinyl-CoA synthetase) is an enzyme in the **Krebs cycle** (citric acid cycle). - It catalyzes the reversible reaction of converting succinyl-CoA to succinate and is not directly part of the electron transport chain. *Incorrect ATP synthase* - **ATP synthase** (Complex V) is the enzyme responsible for synthesizing ATP using the **proton gradient** established by the electron transport chain. - While crucial for oxidative phosphorylation, it acts at the end of the process, utilizing the energy generated, rather than initiating electron transport.

Q: NADH via glycerophosphate shunt makes how many ATP?

2. ***2*** - The **glycerol phosphate shuttle** transfers electrons from **cytosolic NADH** to **FAD** in the mitochondrial electron transport chain. - Each **FADH2** molecule produced then enters the electron transport chain at **Complex II**, ultimately leading to the generation of approximately **2 ATP** molecules. *1* - This option would be correct if the electrons were transferred to a molecule that yields only **one ATP** equivalent, which is not the case for **FADH2**. - No direct mechanism in a shunt generates exactly one ATP per NADH equivalent. *3* - This value represents the ATP yield from **NADH** when it directly enters the electron transport chain via the **malate-aspartate shuttle**, not the **glycerophosphate shuttle**. - The **glycerophosphate shuttle** is less efficient than the **malate-aspartate shuttle**. *4* - This number is not a standard ATP yield for either **NADH** or **FADH2** in the electron transport chain. - The maximum yield for NADH is typically considered to be 2.5 or 3 ATP, and for FADH2 is 1.5 or 2 ATP, depending on the shuttle and precise calculations.

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: What is the role of nonsense codons in protein synthesis?

Termination of protein synthesis. ***Termination of protein synthesis*** - **Nonsense codons**, also known as **stop codons** (UAA, UAG, UGA), signal the end of translation. - When a ribosome encounters a nonsense codon, it binds **release factors** instead of an aminoacyl-tRNA, leading to the dissociation of the polypeptide chain. *Elongation of the polypeptide chain* - **Elongation** involves the sequential addition of amino acids to the growing polypeptide chain, guided by sense codons. - Nonsense codons do not code for any amino acid and thus do not contribute to chain elongation. *Pre-translational modification of proteins* - **Pre-translational modifications** refer to events like protein folding and disulfide bond formation that occur as the polypeptide is being synthesized. - Nonsense codons are involved in halting the synthesis, not in modifying the protein. *Initiation of protein synthesis* - **Initiation** of protein synthesis begins at the **start codon** (AUG), which codes for methionine. - Nonsense codons are distinct from the start codon and fulfill a different role in the translation process.

Q: Progressive distal-to-proximal motor recovery following nerve regeneration is most characteristic of which type of nerve injury?

Axonotmesis. ***Axonotmesis*** - Involves damage to the **axon** and myelin sheath, while the surrounding **epineurium** remains intact. - This preservation of the connective tissue allows for guided **regeneration** of axons from distal to proximal, leading to a good prognosis for recovery [1]. *Neurotmesis* - Refers to the **complete transection** of the nerve, including the axon, myelin, and all connective tissue sheaths. - Recovery is often **incomplete** or requires surgical repair, as significant misdirection of regenerating axons is common. *Neuropraxia* - Characterized by a **temporary block** in nerve conduction, typically due to **demyelination**, with the axon remaining intact. - Recovery is usually **rapid** and complete, occurring within days to weeks, as no axonal regeneration is needed. *Nerve injury* - This is a **general term** that encompasses all types of nerve damage, from mild to severe. - It does not specify a particular mechanism or pattern of recovery, making it less precise than the more specific classifications.

Q: Which isotope is used to measure RBC volume?

Cr 51. ***Cr 51*** - **Chromium-51** attaches irreversibly to the beta chain of hemoglobin, making it an ideal tracer for measuring **red blood cell volume** and survival. - After injection, the labeled red blood cells distribute throughout the circulation, and their dilution allows for the calculation of the total **RBC mass**. *H-3* - **Tritium (H-3)** is typically used as tritiated water to measure **total body water**, as it readily equilibrates throughout all fluid compartments. - It does not specifically bind to red blood cells for mass measurement. *D2O* - **D2O (heavy water)** is used to measure **total body water** content, similar to tritiated water. - It exchanges with water in the body and diffuses into all fluid compartments, rather than targeting red blood cells. *I-135* - While various **iodine isotopes** are used in medicine, such as **I-131** for thyroid imaging or therapy, **I-135** is not a commonly used isotope for measuring red blood cell volume. - Other tracers like **radio-iodinated human serum albumin** (e.g., I-125 HSA) can be used to measure plasma volume, not specifically RBC volume.

Q: The primary site of vasopressin synthesis is

Supraoptic nucleus. ***Supraoptic nucleus*** - The **supraoptic nucleus** of the hypothalamus is the **primary site** for the synthesis of **vasopressin** (also known as antidiuretic hormone or ADH). - Approximately **80% of vasopressin** is produced by the neurosecretory cells in this nucleus. - The synthesized vasopressin is transported down axons to the posterior pituitary for storage and release. *Preoptic nucleus* - The **preoptic nucleus** is involved in various homeostatic functions, including **thermoregulation** and **sleep regulation**, but not the synthesis of vasopressin. - While it has neuronal connections to the hypothalamus, it does not produce ADH. *Paraventricular nucleus* - The **paraventricular nucleus** also synthesizes **both vasopressin and oxytocin**, accounting for approximately **20% of vasopressin production**. - While it does produce vasopressin, the **supraoptic nucleus remains the primary site**, making it the correct answer to this question. - The PVN also plays important roles in stress response and various autonomic functions. *Posterior pituitary* - The **posterior pituitary** (neurohypophysis) is responsible for the **storage and release** of vasopressin and oxytocin, not their synthesis. - These hormones are produced in the hypothalamic nuclei (supraoptic and paraventricular) and then transported down axonal tracts to the posterior pituitary.

Q: Which mechanism is primarily responsible for the transport of glucose in renal tubular cells?

Sodium-glucose cotransport. ***Sodium-glucose cotransport*** - Glucose reabsorption in the renal tubules, particularly in the **proximal tubule**, occurs primarily via **secondary active transport** involving **sodium-glucose cotransporters (SGLTs)**. - SGLT proteins use the **sodium concentration gradient** (maintained by the Na+/K+-ATPase on the basolateral membrane) to move glucose against its concentration gradient from the tubular lumen into the cell. *Glucose diffusion* - While passive diffusion may play a minor role, it is insufficient to reabsorb the large amounts of **filtered glucose** - Diffusion would lead to significant **glucose loss in urine**, even at normal blood glucose levels. *Sodium antiport* - Antiport systems move two different ions or molecules in **opposite directions** across a membrane. - While present in renal cells, sodium antiport mechanisms are not the primary means of **glucose reabsorption**; rather, glucose transport is mostly symport. *Facilitated diffusion* - Facilitated diffusion involves carrier proteins (like **GLUT transporters**) that move molecules down their **concentration gradient**. - While GLUT transporters are present on the **basolateral membrane** of tubular cells to move glucose into the interstitium, they are not the primary mechanism for glucose uptake from the tubular lumen, which occurs against a concentration gradient.

Q: What is the duration of the second heart sound (S2)?

0.08 sec. ***0.08 sec*** - The second heart sound (S2) is composed of two components: A2 (aortic valve closure) and P2 (pulmonic valve closure). The normal duration of S2, encompassing both components, is approximately **0.08 seconds**. - This short duration reflects the rapid closure of the aortic and pulmonic valves at the beginning of **diastole**. *0.15sec* - A duration of **0.15 seconds** for S2 is significantly longer than normal, which could indicate abnormal valve function or conditions causing delayed valve closure. - Such prolonged duration might be observed in conditions like **severe pulmonic stenosis** or **pulmonic hypertension**, which are not the typical duration of a healthy S2. *0.12 sec* - A duration of **0.12 seconds** is also longer than the typical normal range for S2. - While still shorter than 0.15 seconds, it could suggest subtle delays in valve closure or splitting that exceeds the usual physiological splitting. *0.1 sec* - A duration of **0.1 seconds** is slightly prolonged but generally falls within a range that might be considered borderline or indicative of minimal physiological variations. - However, in typical healthy individuals, the S2 duration is closer to 0.08 seconds, making 0.1 seconds less precise for the most common duration.

Question 1

Which enzyme primarily initiates the electron transport process in oxidative phosphorylation?

Accepted Answer

NADH dehydrogenase

Answer

Pyruvate kinase

Answer

Succinyl CoA thiokinase

Answer

ATP synthase

Question 2

NADH via glycerophosphate shunt makes how many ATP?

Accepted Answer

2

Answer

1

Answer

4

Answer

3

Question 3

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 4

What is the role of nonsense codons in protein synthesis?

Accepted Answer

Termination of protein synthesis

Answer

Elongation of the polypeptide chain

Answer

Pre-translational modification of proteins

Answer

Initiation of protein synthesis

Question 5

Progressive distal-to-proximal motor recovery following nerve regeneration is most characteristic of which type of nerve injury?

Accepted Answer

Axonotmesis

Answer

Neurotmesis

Answer

Neuropraxia

Answer

Nerve injury

Question 6

Which isotope is used to measure RBC volume?

Accepted Answer

Cr 51

Answer

H-3

Answer

D2O

Answer

I-135

Question 7

The primary site of vasopressin synthesis is

Accepted Answer

Supraoptic nucleus

Answer

Preoptic nucleus

Answer

Paraventricular nucleus

Answer

Posterior pituitary

Question 8

During moderate exercise, the respiratory rate increases in response to which of the following?

Accepted Answer

Increased PCO2 in arterial blood

Answer

Proprioceptive feedback from muscle spindles

Answer

Decreased PO2 in arterial blood

Answer

Stimulation of J-receptors

Question 9

Which mechanism is primarily responsible for the transport of glucose in renal tubular cells?

Accepted Answer

Sodium-glucose cotransport

Answer

Facilitated diffusion

Answer

Glucose diffusion

Answer

Sodium antiport

Question 10

What is the duration of the second heart sound (S2)?

Accepted Answer

0.08 sec

Answer

0.15 sec

Answer

0.1 sec

Answer

0.12 sec

NEET-PG 2012

Biochemistry

Internal Medicine

Physiology