Acid-Base Chemistry Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Acid-Base Chemistry. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Acid-Base Chemistry Indian Medical PG Question 1: In a patient with a plasma pH of 7.1 the HCO3 / H2CO3 ratio in plasma is:
- A. 1
- B. 20
- C. 2
- D. 10 (Correct Answer)
Acid-Base Chemistry Explanation: ***Correct Answer: 10***
- The **Henderson-Hasselbalch equation** dictates that pH = pKa + log([HCO3-]/[H2CO3]). Given a normal pKa for carbonic acid of 6.1, a pH of 7.1 leads to 7.1 = 6.1 + log([HCO3-]/[H2CO3]), meaning log([HCO3-]/[H2CO3]) = 1, and thus [HCO3-]/[H2CO3] = 10^1 = **10**.
- This ratio of 10 indicates **acidosis**, as the normal physiological ratio for a pH of 7.4 is 20:1.
*Incorrect Option: 1*
- A ratio of 1 ([HCO3-]/[H2CO3] = 1:1) would mean that log(1) = 0, which would result in a pH equal to the pKa, i.e., pH = 6.1. This is an **extremely acidic** condition incompatible with life.
- This ratio would signify a severe and uncompensated metabolic and/or respiratory acidosis.
*Incorrect Option: 20*
- A ratio of 20 ([HCO3-]/[H2CO3] = 20:1) corresponds to a pH of **7.4**, which is the normal physiological pH.
- Since the given plasma pH is 7.1, this ratio is incorrect, as a lower pH indicates a lower ratio.
*Incorrect Option: 2*
- A ratio of 2 ([HCO3-]/[H2CO3] = 2:1) would result in a pH calculation of pH = 6.1 + log(2) = 6.1 + 0.3 = 6.4.
- This pH is also **too low** compared to the given pH of 7.1.
Acid-Base Chemistry Indian Medical PG Question 2: Use the following laboratory values to find the best option that describes the acid-base disorder: Plasma pH = 7.12, Plasma PCO2 = 60 mm Hg, Plasma HCO3- = 19 mEq/L
- A. Combined metabolic and respiratory acidosis (Correct Answer)
- B. Metabolic alkalosis with respiratory compensation
- C. Combined metabolic and respiratory alkalosis
- D. Respiratory acidosis with renal compensation
Acid-Base Chemistry Explanation: ***Combined metabolic and respiratory acidosis***
- The **pH of 7.12** indicates profound **acidemia**, meaning the blood is more acidic than normal.
- The **PCO2 of 60 mm Hg** (normal 35-45 mm Hg) indicates **respiratory acidosis** as the elevated CO2 drives the pH down; the **HCO3- of 19 mEq/L** (normal 22-26 mEq/L) indicates **metabolic acidosis** as the decreased bicarbonate also drives the pH down, making both components contribute to the acidemia.
*Metabolic alkalosis with respiratory compensation*
- This would present with an **elevated pH** (alkalemia) and an **elevated HCO3-**, compensated by an elevated PCO2.
- The given values show a **low pH** and a **low HCO3-**, which contradicts metabolic alkalosis.
*Combined metabolic and respiratory alkalosis*
- This would involve an **elevated pH** with both a **low PCO2** (respiratory alkalosis) and an **elevated HCO3-** (metabolic alkalosis).
- The patient's pH is very low, unequivocally ruling out any form of alkalosis.
*Respiratory acidosis with renal compensation*
- While respiratory acidosis is present due to the high PCO2, the **low bicarbonate (19 mEq/L)** indicates a **metabolic acidosis** rather than renal compensation.
- In compensated respiratory acidosis, the kidneys would retain bicarbonate, leading to an **elevated HCO3-**, which is not seen here.
Acid-Base Chemistry Indian Medical PG Question 3: At pKa = pH, what is the relationship between the ionic and non-ionic forms of a drug?
- A. Absorption of drug is 50% ionic and 50% non-ionic
- B. Conc. of drug is 25% ionic and 75% non-ionic
- C. Conc. of drug is 75% ionic and 25% non-ionic
- D. Conc. of drug is 50% ionic and 50% non-ionic (Correct Answer)
Acid-Base Chemistry Explanation: ***Conc. of drug is 50% ionic and 50% non-ionic***
- At **pKa = pH**, the concentrations of the **ionized** and **unionized** forms of a drug are equal as per the **Henderson-Hasselbalch equation**.
- This means that exactly **half** of the drug molecules are in their charged (ionic) state, and the other half are in their uncharged (non-ionic) state.
*Absorption of drug is 50% ionic and 50% non-ionic*
- The amount of drug that is absorbed is dependent on the **non-ionic concentration** available at the absorption site, but this option incorrectly states that the *absorption itself* is 50% ionic.
- Absorption primarily occurs for the **non-ionic, lipophilic form** as it can more readily cross cell membranes.
*Conc. of drug is 75% ionic and 25% non-ionic*
- This ratio would occur when the **pH** is either 0.5 units above the pKa for a weak acid or 0.5 units below the pKa for a weak base.
- For example, if **pH = pKa + 0.5** (for a weak acid), approximately 75% would be ionic.
*Conc. of drug is 25% ionic and 75% non-ionic*
- This ratio would occur when the **pH** is either 0.5 units below the pKa for a weak acid or 0.5 units above the pKa for a weak base.
- For example, if **pH = pKa - 0.5** (for a weak acid), approximately 25% would be ionic.
Acid-Base Chemistry Indian Medical PG Question 4: Acid-base imbalance is suspected in a patient. Which of the following parameters would you use for initial determination of acid-base status?
- A. pH, PaCO2, and Base excess
- B. pH, PaCO2, and Bicarbonate (Correct Answer)
- C. pH and PaCO2
- D. pH, PaCO2, Bicarbonate, and Base excess
Acid-Base Chemistry Explanation: ***pH, PaCO2, and Bicarbonate***
- The **pH** provides immediate assessment of overall acid-base status (acidemia if <7.35 or alkalemia if >7.45)
- The **PaCO2** reflects the respiratory component - elevated in respiratory acidosis or compensated metabolic alkalosis; decreased in respiratory alkalosis or compensated metabolic acidosis
- The **HCO3- (bicarbonate)** reflects the metabolic component - essential for determining whether the primary disorder is metabolic or respiratory
- This triad forms the **standard approach** to arterial blood gas (ABG) interpretation taught in all major medical textbooks
- Together, these three parameters allow complete initial classification of acid-base disorders using the Henderson-Hasselbalch relationship
*pH and PaCO2*
- While pH and PaCO2 are critical measurements, **without bicarbonate**, you cannot differentiate between metabolic and respiratory disorders or assess metabolic compensation
- For example, a low pH with normal PaCO2 could indicate metabolic acidosis, but you need HCO3- to confirm this diagnosis
- Incomplete for initial acid-base determination
*pH, PaCO2, and Base excess*
- Base excess is a **calculated parameter** used to quantify the metabolic component of acid-base disturbances
- While useful, it is considered a **secondary parameter** for more detailed metabolic analysis rather than essential for initial determination
- Standard ABG interpretation uses bicarbonate, not base excess, as the primary metabolic parameter
*pH, PaCO2, Bicarbonate, and Base excess*
- While this includes all relevant parameters, **base excess is redundant** for initial determination
- Base excess adds quantitative information about metabolic component but is not required for the initial classification of acid-base status
- The essential triad for initial assessment is pH, PaCO2, and HCO3-
Acid-Base Chemistry Indian Medical PG Question 5: Most important buffer system in human blood:
- A. Hemoglobin
- B. Chloride ions
- C. Bicarbonates (Correct Answer)
- D. Phosphate buffer system
Acid-Base Chemistry Explanation: ***Bicarbonates***
- The **bicarbonate buffer system** is the most significant extracellular buffer in human blood due to its high concentration and the ability of its components (CO2 and HCO3-) to be regulated by the lungs and kidneys, respectively.
- It rapidly equilibrates with dissolved CO2, making it highly effective at buffering both acid and base imbalances to maintain **blood pH**.
*Hemoglobin*
- **Hemoglobin** is an important intracellular buffer within red blood cells, primarily buffering carbonic acid formed from CO2 transport.
- While powerful within the red blood cell, it is not the primary buffer system in the overall plasma (extracellular fluid).
*Chloride ions*
- **Chloride ions** are crucial for maintaining electroneutrality in red blood cell buffering processes (e.g., the **chloride shift**), but they do not directly act as a buffer in the traditional sense of accepting or donating protons.
- Their primary role in pH balance is indirect, supporting the function of other buffer systems.
*Phosphate buffer system*
- The **phosphate buffer system** is important, particularly in intracellular fluid and renal tubules, due to its pKa being close to physiological pH.
- However, its concentration in the extracellular fluid (blood plasma) is relatively low compared to bicarbonate, making it less significant for overall blood buffering.
Acid-Base Chemistry Indian Medical PG Question 6: What is the primary mechanism for maintaining acid-base balance during prolonged vomiting?
- A. Increased chloride reabsorption
- B. Increased potassium excretion
- C. Increased bicarbonate excretion (Correct Answer)
- D. Decreased hydrogen secretion
Acid-Base Chemistry Explanation: ***Increased bicarbonate excretion***
- Prolonged vomiting leads to the loss of **gastric acid (HCl)**, causing **metabolic alkalosis**. The kidneys compensate by increasing the excretion of **bicarbonate (HCO3-)** to restore acid-base balance.
- This renal compensation is the primary mechanism to eliminate the excess alkali from the body.
*Increased chloride reabsorption*
- In **metabolic alkalosis** due to vomiting, the body tends to reabsorb less chloride, not more, in an attempt to excrete bicarbonate.
- **Chloride depletion** can actually hinder bicarbonate excretion by promoting sodium reabsorption with bicarbonate.
*Increased potassium excretion*
- **Hypokalemia** can occur with prolonged vomiting due to increased aldosterone activity and direct renal loss associated with metabolic alkalosis.
- However, increased potassium excretion itself is not the primary mechanism for correcting the acid-base disorder; rather, it is a consequence or a contributing factor to the imbalance.
*Decreased hydrogen secretion*
- In response to alkalosis, the kidneys would typically decrease, not increase, **hydrogen ion (H+) secretion** in an effort to retain H+ and normalize pH.
- Decreased H+ secretion is a compensatory mechanism, but the direct excretion of bicarbonate is more crucial for correcting the metabolic alkalosis.
Acid-Base Chemistry Indian Medical PG Question 7: The interpretation of the following ABG value is: pH = 7.5, pCO2 = 50 mm Hg, HCO3 = 30 mEq/L
- A. Respiratory acidosis
- B. Metabolic acidosis
- C. Metabolic alkalosis (Correct Answer)
- D. Normal acid-base balance
Acid-Base Chemistry Explanation: ***Metabolic alkalosis (partially compensated)***
- The **pH of 7.5** indicates **alkalosis**, and the elevated **bicarbonate (HCO3) of 30 mEq/L** is the primary driver of this high pH.
- The elevated **pCO2 of 50 mm Hg** represents **partial respiratory compensation**, where the body retains CO2 to lower the pH toward normal.
- Since the pH remains elevated (not normalized to 7.35-7.45), this is **partially compensated** rather than fully compensated.
*Respiratory acidosis*
- This would be characterized by a **low pH** and an **elevated pCO2**, which is not seen here as the pH is high.
- Although pCO2 is elevated, the **high pH** and **high bicarbonate** rule out primary respiratory acidosis.
*Metabolic acidosis*
- This would present with a **low pH** and a **low bicarbonate** concentration.
- The given values show a **high pH** and **high bicarbonate**, which is the opposite of metabolic acidosis.
*Normal acid-base balance*
- A normal acid-base balance would have a **pH between 7.35-7.45**, a **pCO2 between 35-45 mm Hg**, and an **HCO3 between 22-26 mEq/L**.
- All three values are outside of their normal ranges, indicating an acid-base disturbance.
Acid-Base Chemistry Indian Medical PG Question 8: All of the following statements about acid-base disorders are true, EXCEPT:
- A. Metabolic acidosis is compensated by increasing Pco2 (Correct Answer)
- B. Buffering may be intra & extra cellular
- C. pH determined by Pco2 and HCO3
- D. Respiratory acidosis is compensated by HCO3
Acid-Base Chemistry Explanation: ***Metabolic acidosis is compensated by increasing Pco2***
- In **metabolic acidosis**, the primary problem is a decrease in **bicarbonate (HCO3-)**.
- The compensatory response is **respiratory**, involving an increase in **respiratory rate** and depth to **decrease Pco2**, thereby *raising* the pH back towards normal. Increasing Pco2 would worsen the acidosis.
*Buffering may be intra & extra cellular*
- **Buffering systems** operate both **intracellularly** (e.g., proteins, phosphates) and **extracellularly** (e.g., bicarbonate-carbonic acid system, hemoglobin).
- This dual buffering ensures a rapid and widespread response to changes in acid-base balance throughout the body.
*pH determined by Pco2 and HCO3*
- According to the **Henderson-Hasselbalch equation**, pH is directly proportional to the ratio of **bicarbonate (HCO3-)** to **Pco2**.
- This means that changes in either Pco2 (respiratory component) or HCO3- (metabolic component) will directly influence the overall pH of the blood.
*Respiratory acidosis is compensated by HCO3*
- In **respiratory acidosis**, the primary problem is an increase in **Pco2** due to hypoventilation.
- The compensatory response is **renal**, involving increased reabsorption of **bicarbonate (HCO3-)** and increased excretion of H+ ions to buffer the excess acid.
Acid-Base Chemistry Indian Medical PG Question 9: 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
Acid-Base Chemistry 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.
Acid-Base Chemistry Indian Medical PG Question 10: The lab reports of a patient given below: pH = 7.2, HCO3 = 10 mEq/L, PCO2 = 30 mmHg. This exemplifies which of the following disorders?
- A. Metabolic alkalosis
- B. Respiratory acidosis
- C. Metabolic acidosis (Correct Answer)
- D. Respiratory alkalosis
Acid-Base Chemistry Explanation: ***Metabolic acidosis***
- The pH of 7.2 is acidic, and the **bicarbonate (HCO3) of 10 mEq/L** is significantly low (normal: 22-28 mEq/L), indicating a primary metabolic disturbance causing acidosis.
- The **PCO2 of 30 mmHg** is also low (normal: 35-45 mmHg), which represents **partial respiratory compensation** through hyperventilation to blow off CO2 and raise pH.
- This is a classic example of **metabolic acidosis with respiratory compensation**.
*Metabolic alkalosis*
- This condition would be characterized by a **high pH** and a **high bicarbonate (HCO3)** level, which is the opposite of the given values.
- The body would attempt to compensate by increasing PCO2 through hypoventilation.
*Respiratory acidosis*
- This would present with a **low pH** and a **high PCO2** (>45 mmHg), indicating a primary respiratory problem leading to CO2 retention and acid accumulation.
- Metabolic compensation would show elevated HCO3, not the low HCO3 (10 mEq/L) seen here.
*Respiratory alkalosis*
- This condition is characterized by a **high pH** (>7.45) and a **low PCO2**, due to excessive ventilation causing CO2 elimination.
- While PCO2 is low in the given scenario, the pH is acidic (7.2), not alkalotic, ruling out this diagnosis.
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