Acid-Base Chemistry and Buffers Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Acid-Base Chemistry and Buffers. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Acid-Base Chemistry and Buffers 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 and Buffers 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 and Buffers Indian Medical PG Question 2: A patient with pH of 7, pCO2 of 30 mmHg and Bicarbonate levels of 10 meq. What is the acid base abnormality?
- A. Respiratory alkalosis
- B. Metabolic alkalosis
- C. Respiratory Acidosis
- D. Metabolic Acidosis (Correct Answer)
Acid-Base Chemistry and Buffers Explanation: ***Metabolic Acidosis***
- The pH is 7, which is severely **acidotic** (normal range 7.35-7.45). This indicates an acid-base disorder where the body is too acidic.
- The **bicarbonate level is 10 mEq/L** (normal range 22-26 mEq/L), which is significantly low, directly contributing to the acidosis and pointing towards a metabolic origin.
*Respiratory alkalosis*
- This condition involves an **elevated pH** (alkalosis) due to a primary decrease in pCO2.
- In this case, the pH is acidic, not alkaline.
*Metabolic alkalosis*
- This condition involves an **elevated pH** (alkalosis) due to a primary increase in bicarbonate levels.
- Here, the pH is acidic and bicarbonate is low, directly contradicting metabolic alkalosis.
*Respiratory Acidosis*
- This condition involves a **decreased pH** (acidosis) due to a primary increase in pCO2.
- Although the pH is acidotic, the pCO2 is 30 mmHg (normal range 35-45 mmHg), which is low, indicating a respiratory compensation rather than the primary cause.
Acid-Base Chemistry and Buffers Indian Medical PG Question 3: Which of the following is considered the most important intracellular buffer in human physiology?
- A. Albumin protein
- B. Ammonia buffer
- C. Bicarbonate buffer
- D. Phosphate buffer (Correct Answer)
Acid-Base Chemistry and Buffers Explanation: ***Phosphate buffer***
- The **phosphate buffer system (H₂PO₄⁻/HPO₄²⁻)** is the most important intracellular buffer due to relatively high concentrations of inorganic phosphates within cells
- The pKa₂ of approximately **6.8 is close to intracellular pH** (~7.0-7.2), providing optimal buffering capacity
- Plays a crucial role in buffering acids and bases generated by metabolic processes within cells and is also important in renal tubular buffering
*Albumin protein*
- **Proteins**, including albumin, are important **extracellular buffers** in plasma due to their abundant ionizable amino acid residues
- While proteins do contribute to intracellular buffering (especially hemoglobin in RBCs), the **phosphate system is more significant** for general intracellular pH regulation
*Ammonia buffer*
- The **ammonia buffer system (NH₃/NH₄⁺)** is primarily a **renal buffer system** that plays a crucial role in acid excretion via urine
- It is not considered the primary intracellular buffer for metabolic acid-base balance within cells
*Bicarbonate buffer*
- The **bicarbonate buffer system (HCO₃⁻/H₂CO₃)** is the **most important extracellular buffer system**, critical for maintaining blood pH
- Although present intracellularly, its buffering capacity is less prominent than phosphate within cells due to lower intracellular bicarbonate concentration and its pKa of 6.1 being further from intracellular pH
Acid-Base Chemistry and Buffers Indian Medical PG Question 4: 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 and Buffers 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 and Buffers Indian Medical PG Question 5: The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?
- A. Red blood cell bicarbonate
- B. Extracellular bicarbonate
- C. Plasma proteins
- D. Red blood cell hemoglobin (Correct Answer)
Acid-Base Chemistry and Buffers Explanation: ***Red blood cell hemoglobin***
- **Hemoglobin is the primary buffer** for the massive daily acid load from CO2 (approximately 12,500 mEq H+ per day).
- CO2 diffuses into RBCs where **carbonic anhydrase** rapidly catalyzes: CO2 + H2O → H2CO3 → H+ + HCO3-.
- **Deoxygenated hemoglobin** has a higher affinity for H+ than oxygenated hemoglobin (reduced hemoglobin is a weaker acid, thus better H+ acceptor).
- This buffering is crucial for CO2 transport: **Hb + H+ → HHb**, preventing significant pH changes despite huge CO2 production.
- The bicarbonate produced is then transported out via the **chloride shift** to maintain electrical neutrality.
*Extracellular bicarbonate*
- While the bicarbonate buffer system is quantitatively the largest extracellular buffer, it is **NOT the primary buffer for CO2-derived H+**.
- The extracellular HCO3-/CO2 system primarily buffers **metabolic (non-volatile) acids** produced from dietary and metabolic sources (~50-100 mEq/day).
- For CO2-derived acid, the buffering occurs **intracellularly in RBCs** via hemoglobin before bicarbonate enters the plasma.
*Red blood cell bicarbonate*
- Bicarbonate is produced within RBCs from the dissociation of carbonic acid, but it is **not the buffer itself**.
- The bicarbonate is a **product** of the buffering reaction, not the buffering agent.
- Most RBC-produced HCO3- is transported to plasma via the **anion exchanger (Band 3 protein)** in exchange for Cl-.
*Plasma proteins*
- Plasma proteins like **albumin** have buffering capacity due to ionizable groups (imidazole groups of histidine residues).
- They contribute only about **1-5%** of total blood buffering capacity.
- Far less important than hemoglobin for buffering the large CO2-derived acid load.
Acid-Base Chemistry and Buffers Indian Medical PG Question 6: The primary respiratory compensation for metabolic acidosis is?
- A. HCO3 loss
- B. Cl- loss
- C. Hyperventilation (Correct Answer)
- D. Ammonia excretion in kidney
Acid-Base Chemistry and Buffers Explanation: ***Hyperventilation***
- In **metabolic acidosis**, the body attempts to raise the pH by decreasing the **partial pressure of carbon dioxide (PCO2)**.
- **Hyperventilation** increases the excretion of CO2, a volatile acid, which directly reduces the amount of carbonic acid in the blood and helps to buffer the excess acid.
*HCO3 loss*
- **Bicarbonate (HCO3-) loss** is a cause or consequence of metabolic acidosis, not a compensatory mechanism.
- The kidneys generally try to *retain* or regenerate bicarbonate during acidosis, rather than losing it.
*Cl- loss*
- **Chloride ion (Cl-) loss** is not a primary respiratory compensatory mechanism for metabolic acidosis.
- While shifts in chloride can occur in acid-base imbalances, they are typically related to renal handling or fluid shifts, not direct respiratory compensation.
*Ammonia excretion in kidney*
- **Ammonia excretion** by the kidneys is a renal (kidney) compensatory mechanism, not a respiratory one.
- The kidneys excrete ammonia to excrete hydrogen ions (H+), thereby regenerating bicarbonate and helping to correct the acidosis over a longer period.
Acid-Base Chemistry and Buffers Indian Medical PG Question 7: In plasma, if pH is 5, what is the fraction of base to acid?
- A. 0.01
- B. 0.1 (Correct Answer)
- C. 1
- D. 10
Acid-Base Chemistry and Buffers Explanation: ***0.1***
- This question applies the **Henderson-Hasselbalch equation**: pH = pKa + log([base]/[acid]). For the **bicarbonate buffer system** (the primary plasma buffer), pKa ≈ 6.1.
- Substituting the given values: $5 = 6.1 + \log([HCO_3^-] / [H_2CO_3])$
- Rearranging: $\log([HCO_3^-] / [H_2CO_3]) = 5 - 6.1 = -1.1$
- Therefore: $[HCO_3^-] / [H_2CO_3] = 10^{-1.1} ≈ 0.079$
- Among the given options, **0.079 is closest to 0.1**, making this the correct answer.
- Note: pH 5 in plasma is physiologically impossible (incompatible with life), but this tests theoretical understanding of the buffer equation.
*0.01*
- This ratio would correspond to an even **more acidic** condition with $\log([base]/[acid]) = -2$.
- Using Henderson-Hasselbalch: pH = 6.1 + (-2) = 4.1, which is lower than the given pH of 5.
- The calculated ratio of 0.079 is much closer to 0.1 than to 0.01.
*1*
- A ratio of 1 means **equal concentrations** of base and acid, which occurs when pH = pKa.
- This would give pH = 6.1, not the given pH of 5.
- This represents a **neutral buffer condition**, not the acidic state described.
*10*
- This ratio indicates an **alkaline** solution with 10 times more base than acid.
- Using Henderson-Hasselbalch: pH = 6.1 + log(10) = 6.1 + 1 = 7.1 (physiological alkalosis).
- This contradicts the given acidic pH of 5.
Acid-Base Chemistry and Buffers Indian Medical PG Question 8: Which one of the following biochemical abnormalities can be produced by repeated vomiting?
- A. Metabolic acidosis
- B. Metabolic alkalosis (Correct Answer)
- C. Ketosis
- D. Uraemia
Acid-Base Chemistry and Buffers Explanation: ***Metabolic alkalosis***
- Repeated vomiting leads to the loss of **hydrochloric acid (HCl)** from the stomach, causing **hypochloremic metabolic alkalosis** with an increase in serum **bicarbonate (HCO3-)** and a rise in blood pH.
- The loss of H+ and Cl- ions results in **compensatory hypokalemia** as the kidneys exchange K+ for H+ to maintain electroneutrality.
- **Volume depletion** from vomiting triggers aldosterone secretion, which further promotes K+ loss and H+ excretion, perpetuating the alkalosis (contraction alkalosis).
- This is one of the most common causes of metabolic alkalosis in clinical practice.
*Metabolic acidosis*
- This condition is characterized by a decrease in **serum pH** and **bicarbonate levels**, typically due to excess acid production or bicarbonate loss from diarrhea or renal tubular acidosis.
- Vomiting does not directly cause metabolic acidosis; rather, it leads to the opposite effect by removing acidic gastric contents.
*Ketosis*
- **Ketosis** occurs when the body metabolizes fat for energy, producing **ketone bodies**, common in conditions like uncontrolled diabetes or prolonged starvation.
- While severe, prolonged vomiting with reduced oral intake can indirectly lead to starvation ketosis, the primary and most characteristic biochemical abnormality of repeated vomiting is metabolic alkalosis, not ketosis.
*Uraemia*
- **Uraemia** is a syndrome caused by the accumulation of **nitrogenous waste products** (urea, creatinine) in the blood, primarily due to kidney failure.
- Vomiting may be a *symptom* of uraemia, but it does not *cause* uraemia. Kidney function is the primary determinant of urea levels.
Acid-Base Chemistry and Buffers Indian Medical PG Question 9: Increased serum calcium is seen in all conditions except:
- A. Myxedema (Correct Answer)
- B. Multiple myeloma
- C. Sarcoidosis
- D. Primary hyperparathyroidism
Acid-Base Chemistry and Buffers Explanation: ### Explanation
**Correct Answer: A. Myxedema**
**1. Why Myxedema is the correct answer:**
Myxedema refers to severe **hypothyroidism**. In this condition, serum calcium levels are typically **normal or slightly decreased**, but never increased. Thyroid hormones normally stimulate bone resorption; therefore, in a hypothyroid state, there is a decrease in bone turnover. In contrast, it is *Hyperthyroidism* that is occasionally associated with mild hypercalcemia due to increased osteoclastic activity.
**2. Analysis of Incorrect Options (Causes of Hypercalcemia):**
* **Multiple Myeloma:** This is a plasma cell dyscrasia where malignant cells produce "Osteoclast Activating Factors" (like IL-6 and TNF-beta). This leads to extensive bone resorption (punched-out lesions) and significant hypercalcemia.
* **Sarcoidosis:** This granulomatous disease involves macrophages that express the enzyme **1-alpha-hydroxylase**. This enzyme converts Vitamin D to its active form (1,25-dihydroxyvitamin D), leading to increased intestinal calcium absorption and hypercalcemia.
* **Primary Hyperparathyroidism:** Usually caused by a parathyroid adenoma, it results in excessive secretion of Parathyroid Hormone (PTH). PTH increases bone resorption, renal calcium reabsorption, and intestinal absorption (via Vitamin D activation), making it the most common cause of hypercalcemia in outpatient settings.
**3. NEET-PG High-Yield Pearls:**
* **Most common cause of hypercalcemia (Outpatient):** Primary Hyperparathyroidism.
* **Most common cause of hypercalcemia (Inpatient/Hospitalized):** Malignancy.
* **Milk-Alkali Syndrome:** A classic triad of hypercalcemia, metabolic alkalosis, and renal failure due to excessive ingestion of calcium carbonate.
* **ECG Finding:** Hypercalcemia causes a **shortened QT interval**, whereas hypocalcemia causes a prolonged QT interval.
Acid-Base Chemistry and Buffers Indian Medical PG Question 10: Which is the most effective buffer system in the blood that is controlled by respiration?
- A. Bicarbonates (Correct Answer)
- B. Hemoglobin
- C. Proteins
- D. Phosphates
Acid-Base Chemistry and Buffers Explanation: **Explanation:**
The **Bicarbonate buffer system ($HCO_3^- / CO_2$)** is the most important and effective extracellular buffer in the blood. Its effectiveness stems from being an **"open system."** Unlike other buffers, its components are independently regulated by two major organs: the **lungs** (controlling $CO_2$ via respiration) and the **kidneys** (controlling $HCO_3^-$ excretion and reabsorption). According to the Henderson-Hasselbalch equation, the pH of blood is determined by the ratio of bicarbonate to dissolved $CO_2$. By increasing or decreasing the rate of respiration (ventilation), the body can rapidly adjust $pCO_2$ levels to maintain this ratio, making it the primary respiratory-controlled buffer.
**Analysis of Incorrect Options:**
* **Hemoglobin (B):** While hemoglobin is a powerful buffer (due to histidine residues) and is the most important buffer **inside erythrocytes**, it is not primarily controlled by respiration; it depends on the oxygenation state (Bohr effect).
* **Proteins (C):** Plasma proteins (like albumin) act as buffers in the blood, but their concentration remains relatively static and is not acutely regulated by respiratory changes.
* **Phosphates (D):** The phosphate buffer system is crucial **intracellularly** and in the **renal tubules** (where its pKa of 6.8 is close to tubular pH). However, its concentration in the plasma is too low to be the most effective blood buffer.
**High-Yield Clinical Pearls for NEET-PG:**
* **Normal $HCO_3^- : CO_2$ ratio:** 20:1 (maintains physiological pH of 7.4).
* **First line of defense:** Chemical buffers (seconds).
* **Second line of defense:** Respiratory system (minutes).
* **Third line of defense:** Renal system (hours to days).
* **Isohydric Principle:** All buffer systems in the body are in equilibrium with each other; a change in one affects all others.
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