General Physiology Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for General Physiology. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
General Physiology Indian Medical PG Question 1: What is the second messenger responsible for smooth muscle relaxation mediated by nitric oxide (NO)?
- A. Calcium
- B. Cyclic AMP
- C. Cyclic GMP (Correct Answer)
- D. Magnesium
General Physiology Explanation: ***Cyclic GMP***
- **Nitric oxide (NO)** activates **guanylyl cyclase**, an enzyme that converts **GTP to cGMP**.
- Elevated **cGMP** levels activate **protein kinase G (PKG)**, leading to smooth muscle relaxation through various mechanisms, including reduced intracellular calcium and altered sensitivity of contractile proteins.
*Calcium*
- **Calcium** is primarily a key second messenger for **smooth muscle contraction**, not relaxation.
- An increase in intracellular **calcium** promotes the binding of **calcium to calmodulin**, activating myosin light chain kinase and leading to contraction.
*Cyclic AMP*
- While **cyclic AMP (cAMP)** can cause smooth muscle relaxation (e.g., via beta-2 adrenergic stimulation), it is not the direct second messenger for **nitric oxide (NO)**-mediated relaxation.
- **cAMP** is produced by **adenylyl cyclase** and primarily activates **protein kinase A (PKA)**.
*Magnesium*
- **Magnesium** is an important cofactor for many enzymes and can influence muscle contraction and relaxation, but it does not serve as a primary second messenger for **nitric oxide (NO)**.
- High concentrations of **magnesium** can directly induce muscle relaxation by competing with **calcium** and modulating various channels and enzymes.
General Physiology Indian Medical PG Question 2: All are examples of negative feedback except
- A. Regulation of blood CO2 level
- B. Regulation of pituitary hormones
- C. Regulation of blood pressure
- D. Coagulation of the blood (Correct Answer)
General Physiology Explanation: ***Coagulation of the blood***
- **Blood coagulation** is a classic example of **positive feedback**, where the initial clotting process amplifies itself until bleeding stops
- Platelets aggregate and release factors that promote further platelet aggregation and activation of the clotting cascade, thereby **accelerating the response** rather than diminishing it
- This is the exception among the options, as it represents positive feedback while all others are negative feedback
*Regulation of blood CO2 level*
- The regulation of **blood CO2 levels** is a vital example of **negative feedback**, where an increase in CO2 stimulates breathing to expel excess CO2
- This mechanism works to return the blood CO2 concentration to its homeostatic set point, thus **counteracting the initial stimulus**
- Central and peripheral chemoreceptors detect elevated CO2 and trigger increased ventilation
*Regulation of pituitary hormones*
- The regulation of **pituitary hormones** involves **negative feedback loops**, where high levels of target gland hormones inhibit the release of stimulating hormones from the pituitary and hypothalamus
- For example, high thyroid hormone levels inhibit TSH release from the pituitary and TRH from the hypothalamus
- This effectively **reduces the initial stimulus** and maintains hormonal balance
*Regulation of blood pressure*
- The regulation of **blood pressure** is primarily controlled by **negative feedback mechanisms** involving baroreceptors, which detect changes in pressure
- If blood pressure rises, baroreceptors in the carotid sinus and aortic arch signal the medulla to reduce heart rate and dilate blood vessels
- This response **lowers the pressure back to the set point**, maintaining cardiovascular homeostasis
General Physiology Indian Medical PG Question 3: Which of the following statements best describes the mechanism of action of insulin on target cells?
- A. Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.
- B. Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.
- C. Insulin enters the cell and causes the release of calcium ions from intracellular stores.
- D. Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor. (Correct Answer)
General Physiology Explanation: ***Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor.***
- **Insulin** is a **peptide hormone** and cannot freely pass through the lipid bilayer, thus it binds to a **transmembrane receptor** on the cell surface.
- This binding leads to the activation of the receptor's intrinsic **tyrosine kinase activity** in the intracellular domain, initiating a signaling cascade.
*Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.*
- This mechanism describes the action of **steroid hormones**, which are lipid-soluble and can cross the cell membrane, binding to **intracellular receptors**.
- **Insulin** acts via a **cell surface receptor** and its downstream effects are mediated through signal transduction pathways, not direct nuclear translocation.
*Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.*
- This mechanism is characteristic of **G-protein coupled receptors (GPCRs)**, which activate or inhibit enzymes like adenylate cyclase via G-proteins to produce second messengers like cyclic AMP.
- The **insulin receptor** is a **receptor tyrosine kinase**, not a GPCR, and does not directly activate adenylate cyclase via Gs protein.
*Insulin enters the cell and causes the release of calcium ions from intracellular stores.*
- While some hormones and neurotransmitters can trigger the release of intracellular **calcium ions**, this is typically mediated by specific pathways (e.g., GPCRs linked to phospholipase C).
- **Insulin** does not directly enter target cells to cause calcium release; its actions are primarily mediated through receptor tyrosine kinase signaling pathways.
General Physiology Indian Medical PG Question 4: All are true regarding Sunitinib except which of the following?
- A. It inhibits tyrosine kinase receptors
- B. It is excreted primarily in urine (Correct Answer)
- C. It is used for the treatment of GIST
- D. It is used for renal cell carcinoma
General Physiology Explanation: ***It is excreted primarily in urine***
- **Sunitinib** is predominantly metabolized in the **liver** by CYP3A4 and primarily excreted in the **feces**, not urine.
- Its major active metabolite, N-desethyl sunitinib, is also primarily eliminated via the fecal route.
*It inhibits tyrosine kinase receptors*
- **Sunitinib** is a **multitargeted receptor tyrosine kinase (RTK) inhibitor**.
- It blocks several RTKs involved in tumor growth, angiogenesis, and metastatic progression, such as **VEGFR, PDGFR, KIT, and FLT3**.
*It is used for the treatment of GIST*
- **Sunitinib** is approved for the treatment of **imatinib-refractory** or **imatinib-intolerant gastrointestinal stromal tumors (GIST)**.
- Its mechanism in GIST involves inhibiting KIT and PDGFR, which are often mutated and constitutively active in this cancer.
*It is used for renal cell carcinoma*
- **Sunitinib** is a standard first-line treatment for **advanced renal cell carcinoma (RCC)**.
- Its efficacy in RCC is primarily due to its inhibition of VEGFR, which targets the high vascularity characteristic of kidney tumors.
General Physiology Indian Medical PG Question 5: HCO3/H2CO3 is the best buffer because it is:
- A. Its components can be increased or decreased in the body as needed (Correct Answer)
- B. Good acceptor and donor of H+ ions
- C. Combination of a weak acid and weak base
- D. pKa near physiological pH
General Physiology Explanation: ***Its components can be increased or decreased in the body as needed***
- The **bicarbonate buffer system** is unique because its components, **bicarbonate (HCO3-)** and **carbon dioxide (CO2)**, are physiologically regulated by the kidneys and lungs, respectively.
- This allows for dynamic adjustment of buffer concentrations to maintain **pH homeostasis**, making it highly effective even when its pKa is not perfectly matched to physiological pH.
*Good acceptor and donor of H+ ions*
- While bicarbonate acts as an **acceptor of H+ ions** and carbonic acid can donate H+ ions, this characteristic is true for all effective buffer systems.
- This option does not highlight the unique advantage of the bicarbonate buffer over other physiological buffers.
*Combination of a weak acid and weak base*
- The bicarbonate buffer system indeed consists of **carbonic acid (H2CO3)**, a weak acid, and its conjugate base, **bicarbonate (HCO3-)**.
- However, this is the definition of any buffer system and doesn't explain why it's the *best* physiological buffer compared to others.
*pKa near physiological pH*
- The **pKa of the bicarbonate buffer system is 6.1**, which is not exactly at the physiological pH of 7.4.
- While buffers are generally most effective when their pKa is close to the pH they regulate, the **open nature and physiological regulation** of the bicarbonate system compensate for this difference.
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