Acid-Base Balance — MCQs

A 43-year-old woman develops acute renal failure following an emergency resection of a leaking abdominal aortic aneurysm. One week after surgery, the following laboratory values are obtained: Serum electrolytes (mEq/L): Na+ 127, K+ 5.9, Cl- 92, HCO3- 15. Blood urea nitrogen: 82 mg/dL. Serum creatinine: 6.7 mg/dL. The patient has gained 4 kg since surgery and is mildly dyspneic at rest. Eight hours after these values are reported, the following electrocardiogram is obtained. Which of the following is the most appropriate initial treatment in the management of this patient?

Q19Easy

Which of the following is the primary acid-base disorder associated with hemorrhagic shock, diabetic ketoacidosis, and chronic renal failure?

Q20Easy

What is the normal value of arterial HCO3' in gas exchange?

Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 11: Which of the following is true about the Anion Gap?

A. It is the difference between unmeasured anions and unmeasured cations. (Correct Answer)
B. The normal value is 20 mEq/L.
C. The anion gap is increased in diarrhea.
D. The anion gap increases if unmeasured cations increase.

Explanation: **Explanation:** The **Anion Gap (AG)** is a clinical tool used to differentiate causes of metabolic acidosis. It is based on the principle of **electroneutrality**: the total number of positive charges (cations) must equal the total number of negative charges (anions) in the serum. **1. Why Option A is Correct:** The AG represents the "gap" between measured cations and measured anions. Mathematically, it is expressed as: $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$ Since total cations must equal total anions, this gap is mathematically equivalent to the difference between **unmeasured anions** (e.g., albumin, phosphate, sulfate, organic acids) and **unmeasured cations** (e.g., $K^+$, $Ca^{2+}$, $Mg^{2+}$). **2. Why the Other Options are Incorrect:** * **Option B:** The normal range for the anion gap is typically **8–12 mEq/L** (or 10–14 mEq/L depending on the lab). A value of 20 mEq/L is significantly elevated. * **Option C:** Diarrhea causes a **Normal Anion Gap Metabolic Acidosis (NAGMA)**. In diarrhea, $HCO_3^-$ is lost and replaced by $Cl^-$ (hyperchloremic acidosis), keeping the AG within the normal range. * **Option D:** If unmeasured cations (like $K^+$ or $Mg^{2+}$) increase, the gap between measured ions actually **decreases**. Conversely, a decrease in unmeasured anions (like hypoalbuminemia) also lowers the AG. **High-Yield Clinical Pearls for NEET-PG:** * **MUDPILES:** Mnemonic for High Anion Gap Metabolic Acidosis (HAGMA) — Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates. * **Albumin Correction:** Albumin is the major unmeasured anion. For every **1 g/dL decrease** in serum albumin below 4 g/dL, the "normal" AG decreases by approximately **2.5 mEq/L**. * **NAGMA Causes:** Diarrhea, Renal Tubular Acidosis (RTA), and Acetazolamide use.

Question 12: Features seen in a patient with chronic vomiting are all EXCEPT:

A. Hyponatremia
B. Hypochloremia
C. Metabolic acidosis (Correct Answer)
D. Hypokalemia

Explanation: **Explanation:** Chronic vomiting leads to the loss of gastric contents, primarily hydrochloric acid (HCl), which results in **Metabolic Alkalosis**, not acidosis. This is the classic "Contraction Alkalosis" seen in clinical practice. **1. Why Metabolic Acidosis is the Correct Answer (The Exception):** Vomiting causes a significant loss of hydrogen ions ($H^+$) from the stomach. As $H^+$ is lost, the body generates bicarbonate ($HCO_3^-$) to compensate, leading to an increase in blood pH. Therefore, the patient develops **Metabolic Alkalosis**. Acidosis would only occur in cases of severe lower GI loss (like diarrhea). **2. Analysis of Other Options:** * **Hypochloremia (B):** Gastric juice is rich in Chloride ($Cl^-$). Persistent vomiting leads to direct depletion of chloride, causing hypochloremic alkalosis. * **Hyponatremia (A):** Loss of gastric fluid leads to ECF volume depletion. This triggers the release of ADH (Antidiuretic Hormone), which causes water retention, and Aldosterone, which attempts to save sodium but is often overwhelmed by the fluid loss, resulting in net hyponatremia. * **Hypokalemia (D):** This occurs due to two reasons: direct loss in vomitus and, more importantly, renal compensation. In an attempt to conserve $H^+$ and volume, the kidneys exchange Potassium ($K^+$) for Sodium under the influence of Aldosterone, leading to urinary potassium wasting. **High-Yield Clinical Pearls for NEET-PG:** * **Paradoxical Aciduria:** In severe vomiting, despite systemic alkalosis, the urine becomes acidic. This happens because the kidney prioritizes volume expansion (reabsorbing $Na^+$) over pH balance, excreting $H^+$ instead of $K^+$ once $K^+$ stores are depleted. * **Formula:** Chronic vomiting = Hypokalemic, Hypochloremic, Metabolic Alkalosis with Paradoxical Aciduria.

Question 13: Hypokalemia is frequently associated with which acid-base disorder?

A. Metabolic acidosis
B. Metabolic alkalosis (Correct Answer)
C. Respiratory acidosis
D. Respiratory alkalosis

Explanation: **Explanation:** The relationship between potassium and acid-base balance is governed by the **H⁺-K⁺ exchange mechanism** across cell membranes. **Why Metabolic Alkalosis is Correct:** Hypokalemia and metabolic alkalosis often coexist due to two primary mechanisms: 1. **Transcellular Shift:** When extracellular potassium is low, K⁺ moves out of cells to maintain plasma levels. To maintain electroneutrality, H⁺ ions move from the extracellular fluid into the cells. This loss of extracellular H⁺ results in alkalosis. 2. **Renal Compensation:** In the distal convoluted tubule and collecting ducts, the body attempts to conserve K⁺. However, to reabsorb Na⁺, the kidneys must secrete either K⁺ or H⁺. In hypokalemia, K⁺ is unavailable for secretion, so the kidney secretes H⁺ instead. This leads to **"paradoxical aciduria"** and worsens the systemic alkalosis. **Why Other Options are Incorrect:** * **Metabolic Acidosis:** Typically associated with **hyperkalemia**. As extracellular H⁺ increases, it moves into cells, forcing K⁺ out into the plasma. * **Respiratory Acidosis/Alkalosis:** These are primarily driven by CO₂ retention or washout. While chronic respiratory disorders can cause secondary electrolyte shifts, the classic, high-yield association for hypokalemia is metabolic alkalosis (e.g., in vomiting or diuretic use). **NEET-PG High-Yield Pearls:** * **"Alkalosis causes Hypokalemia; Acidosis causes Hyperkalemia"** (Exception: Diarrhea and Renal Tubular Acidosis, where both acidosis and hypokalemia occur). * **Vomiting:** Leads to metabolic alkalosis, hypochloremia, and hypokalemia. * **Aldosterone:** Increases Na⁺ reabsorption while promoting both K⁺ and H⁺ excretion, leading to hypokalemic metabolic alkalosis (e.g., Conn’s Syndrome).

Question 14: A 40-year-old male presents with excessive hyperventilation. Arterial blood gas analysis reveals pH 7.5, PCO2 24 mmHg, and PO2 88 mm of Hg. What is the most likely diagnosis based on these findings?

A. Respiratory alkalosis (Correct Answer)
B. Metabolic alkalosis
C. Respiratory acidosis
D. Metabolic acidosis

Explanation: ### Explanation **1. Why Respiratory Alkalosis is Correct:** The diagnosis of any acid-base disorder follows a systematic three-step approach: * **Step 1 (pH):** The normal arterial pH is 7.35–7.45. A pH of **7.5** indicates **alkalemia**. * **Step 2 (Primary Cause):** Look at the $PCO_2$ (Normal: 35–45 mmHg). Here, the $PCO_2$ is **24 mmHg** (low). Since $CO_2$ acts as an acid, a decrease in $PCO_2$ raises the pH. * **Step 3 (Correlation):** The patient is hyperventilating. Hyperventilation "washes out" $CO_2$, leading to a primary decrease in $PaCO_2$ and a subsequent rise in pH. This confirms **Respiratory Alkalosis**. **2. Why the Other Options are Incorrect:** * **Metabolic Alkalosis:** This would present with an elevated pH but a primary **increase in $HCO_3^-$** (bicarbonate), not a primary decrease in $PCO_2$. * **Respiratory Acidosis:** This occurs when there is hypoventilation (e.g., COPD, opioid overdose), leading to $CO_2$ retention. The pH would be **< 7.35** and $PCO_2$ would be **> 45 mmHg**. * **Metabolic Acidosis:** This is characterized by a **low pH (< 7.35)** and a primary **decrease in $HCO_3^-$**. **3. NEET-PG High-Yield Pearls:** * **Common Causes:** Anxiety/Panic attacks (most common), high altitude (hypoxia-induced hyperventilation), pulmonary embolism, and early salicylate poisoning. * **Compensation:** In acute respiratory alkalosis, for every 10 mmHg drop in $PCO_2$, the $HCO_3^-$ drops by **2 mEq/L**. In chronic cases, it drops by **4–5 mEq/L**. * **Clinical Sign:** Watch for **hypocalcemia symptoms** (tetany, carpopedal spasm) because alkalosis increases the binding of calcium to albumin, reducing ionized calcium levels.

Question 15: How long does it take for metabolic compensation to occur in a child with respiratory acidosis?

A. Less than 1 day
B. 1-2 days
C. 3-4 days (Correct Answer)
D. More than 7 days

Explanation: **Explanation:** The correct answer is **C (3-4 days)**. **1. Underlying Medical Concept:** Acid-base compensation occurs via two primary systems: the lungs (respiratory) and the kidneys (metabolic). In respiratory acidosis, the primary defect is CO₂ retention. To compensate, the kidneys must increase the reabsorption of bicarbonate ($HCO_3^-$) and the excretion of hydrogen ions ($H^+$). Unlike respiratory compensation (which begins within minutes), **renal compensation is a slow process**. It takes approximately 6–12 hours to begin, but requires **3 to 5 days (average 3-4 days)** to reach its maximal effectiveness and achieve a steady state. **2. Analysis of Incorrect Options:** * **Option A (< 1 day):** This is too short. Within the first 24 hours, only minimal "acute" cellular buffering occurs; the kidneys have not yet significantly altered plasma bicarbonate levels. * **Option B (1-2 days):** While renal mechanisms have started, they are still in the early phase and have not reached the full compensatory capacity required to stabilize the pH. * **Option D (> 7 days):** Compensation is usually maximal by day 5. If the pH is not compensated by a week, it suggests either a very severe primary insult or a secondary renal pathology preventing compensation. **3. NEET-PG High-Yield Pearls:** * **Acute vs. Chronic:** Respiratory acidosis is classified as "Chronic" only after renal compensation has occurred (typically >3 days). * **The Rule of Thumb:** * **Acute Respiratory Acidosis:** $HCO_3^-$ increases by **1 mEq/L** for every 10 mmHg rise in $PaCO_2$. * **Chronic Respiratory Acidosis:** $HCO_3^-$ increases by **3.5 to 4 mEq/L** for every 10 mmHg rise in $PaCO_2$. * **Speed of Compensation:** Respiratory compensation for metabolic disorders is **fast** (minutes to hours); Metabolic compensation for respiratory disorders is **slow** (days).

Question 16: Which of the following is the principal buffer in interstitial fluid?

A. Hemoglobin
B. Other proteins
C. H2CO3 (Correct Answer)
D. H2PO4

Explanation: **Explanation:** The acid-base balance of the body is maintained by various buffer systems distributed across different fluid compartments. The **Bicarbonate Buffer System ($H_2CO_3 / HCO_3^-$)** is the most important and principal buffer in the **extracellular fluid (ECF)**, which includes both plasma and **interstitial fluid**. **Why $H_2CO_3$ is the Correct Answer:** The interstitial fluid is essentially an ultrafiltrate of plasma. It is rich in bicarbonate but notably **lacks significant amounts of proteins**. Therefore, the $H_2CO_3 / HCO_3^-$ system becomes the primary defense against pH changes in this compartment. It is highly effective because its components are regulated by the lungs ($CO_2$) and the kidneys ($HCO_3^-$). **Analysis of Incorrect Options:** * **A. Hemoglobin:** This is a major buffer, but it is located exclusively **inside erythrocytes** (intracellular). It is not present in the interstitial fluid. * **B. Other proteins:** While proteins are the most abundant buffers **intracellularly** and are present in plasma (e.g., Albumin), the interstitial fluid has a very low protein concentration, making them minor contributors here. * **D. $H_2PO_4$ (Phosphate Buffer):** This is the principal **intracellular** buffer and a major urinary buffer. Its concentration in the ECF/interstitial fluid is too low to be the "principal" buffer. **NEET-PG High-Yield Pearls:** * **Principal ECF Buffer:** Bicarbonate system ($H_2CO_3$). * **Principal ICF Buffer:** Proteins and Phosphates. * **Principal Buffer in RBCs:** Hemoglobin. * **First line of defense** against pH changes: Chemical buffers (seconds). * **Second line:** Respiratory system (minutes). * **Third line (most powerful):** Renal system (hours to days).

Question 17: An arterial blood gas (ABG) of a patient shows decreased pH, increased pCO2, and high bicarbonate. What is the most likely diagnosis?

A. Respiratory alkalosis, compensated
B. Respiratory acidosis, compensated
C. Respiratory acidosis, not fully compensated (Correct Answer)
D. Metabolic alkalosis, uncompensated

Explanation: **Explanation:** To solve acid-base problems, follow a systematic three-step approach: 1. **pH Status:** The pH is **decreased (<7.35)**, indicating **Acidosis**. 2. **Primary Cause:** The **pCO2 is increased (>45 mmHg)**. Since CO2 is an acid, its elevation explains the low pH, confirming the primary disorder is **Respiratory Acidosis**. 3. **Compensation:** The **Bicarbonate (HCO3-) is high (>26 mEq/L)**. In respiratory acidosis, the kidneys compensate by retaining bicarbonate to buffer the excess acid. However, because the **pH is still abnormal (low)**, the compensation is partial/incomplete. If it were "fully compensated," the pH would be back within the normal range (7.35–7.45). **Analysis of Incorrect Options:** * **Option A:** Incorrect because the pH is low (acidosis), not high (alkalosis). * **Option B:** In "fully compensated" respiratory acidosis, the pH must be within the normal range (7.35–7.40). Here, the pH is overtly decreased. * **Option D:** Metabolic alkalosis would present with a high pH and high HCO3-. **NEET-PG High-Yield Pearls:** * **ROME Mnemonic:** **R**espiratory **O**pposite (pH ↓, pCO2 ↑ or vice versa), **M**etabolic **E**qual (pH ↑, HCO3- ↑ or vice versa). * **Compensation Speed:** Respiratory compensation (via lungs) is rapid (minutes to hours), while metabolic compensation (via kidneys) is slow (2–5 days). * **Chronic vs. Acute:** A high bicarbonate in the setting of respiratory acidosis usually suggests a **chronic** process (e.g., COPD), as the kidneys require time to elevate HCO3- levels.

Question 18: A 43-year-old woman develops acute renal failure following an emergency resection of a leaking abdominal aortic aneurysm. One week after surgery, the following laboratory values are obtained: Serum electrolytes (mEq/L): Na+ 127, K+ 5.9, Cl- 92, HCO3- 15. Blood urea nitrogen: 82 mg/dL. Serum creatinine: 6.7 mg/dL. The patient has gained 4 kg since surgery and is mildly dyspneic at rest. Eight hours after these values are reported, the following electrocardiogram is obtained. Which of the following is the most appropriate initial treatment in the management of this patient?

A. 10 mL of 10% calcium gluconate (Correct Answer)
B. 0.25 mg digoxin every 3 hours for 3 doses
C. 100 mg lidocaine
D. Emergent hemodialysis

Explanation: ***10 mL of 10% calcium gluconate*** - **Hyperkalemia** (K+ 5.9 mEq/L) with ECG changes requires immediate **membrane stabilization** with calcium gluconate to prevent fatal arrhythmias. - Calcium gluconate is the **first-line treatment** for hyperkalemia with cardiac manifestations, stabilizing the cardiac membrane within minutes. *0.25 mg digoxin every 3 hours for 3 doses* - **Digoxin** is contraindicated in hyperkalemia as it can worsen **cardiac toxicity** and increase risk of arrhythmias. - High potassium levels increase sensitivity to digoxin, making this treatment potentially **life-threatening**. *100 mg lidocaine* - **Lidocaine** is used for ventricular arrhythmias but does not address the underlying **hyperkalemia** causing the ECG changes. - It provides no **membrane stabilization** against potassium-induced cardiac toxicity and delays appropriate treatment. *Emergent hemodialysis* - While **hemodialysis** is necessary for definitive potassium removal, it takes time to initiate and does not provide **immediate cardiac protection**. - **Membrane stabilization** with calcium must come first, followed by measures to shift potassium intracellularly, then removal via dialysis.

Question 19: Which of the following is the primary acid-base disorder associated with hemorrhagic shock, diabetic ketoacidosis, and chronic renal failure?

A. Metabolic acidosis (Correct Answer)
B. Metabolic alkalosis
C. Respiratory acidosis
D. Respiratory alkalosis

Explanation: **Explanation:** The primary acid-base disorder common to all three conditions is **Metabolic Acidosis**, characterized by a primary decrease in plasma bicarbonate ($HCO_3^-$) and a decrease in arterial pH. * **Hemorrhagic Shock:** Leads to decreased tissue perfusion and hypoxia. This shifts metabolism from aerobic to anaerobic, resulting in the accumulation of **Lactic Acid** (High Anion Gap Metabolic Acidosis - HAGMA). * **Diabetic Ketoacidosis (DKA):** Insulin deficiency leads to the breakdown of fatty acids into **Ketoacids** (acetoacetate and $\beta$-hydroxybutyrate), which consume bicarbonate buffers. * **Chronic Renal Failure:** The kidneys fail to excrete the daily fixed acid load (phosphates/sulfates) and show impaired regeneration of bicarbonate, leading to systemic acid accumulation. **Why other options are incorrect:** * **Metabolic Alkalosis:** Occurs due to loss of $H^+$ (e.g., vomiting) or gain of $HCO_3^-$. None of the listed conditions involve acid loss. * **Respiratory Acidosis:** Caused by alveolar hypoventilation and $CO_2$ retention (e.g., COPD, opioid overdose). In the listed conditions, the respiratory system actually compensates by *increasing* ventilation (Kussmaul breathing) to blow off $CO_2$. * **Respiratory Alkalosis:** Caused by hyperventilation (e.g., high altitude, anxiety). While it may occur as a compensatory mechanism in these states, it is not the *primary* disorder. **High-Yield Clinical Pearls for NEET-PG:** * **Anion Gap (AG):** Always calculate AG in metabolic acidosis. DKA, Lactic acidosis (Shock), and Renal failure are classic causes of **HAGMA** (Mnemonic: MUDPILES). * **Kussmaul Respiration:** Deep, rapid breathing is a hallmark compensatory sign of severe metabolic acidosis (especially DKA). * **Winter’s Formula:** Used to calculate expected $pCO_2$ compensation: $pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$.

Question 20: What is the normal value of arterial HCO3' in gas exchange?

A. 40-45 meq/L
B. 20-24 meq/L (Correct Answer)
C. 10-15 meq/L
D. 5-10 meq/L

Explanation: **Explanation:** The normal concentration of bicarbonate ($HCO_3^-$) in arterial blood is a critical component of the body's acid-base buffering system, specifically the **Bicarbonate-Carbonic Acid Buffer System**. According to the Henderson-Hasselbalch equation, the pH of arterial blood (normal range 7.35–7.45) is determined by the ratio of bicarbonate (regulated by the kidneys) to partial pressure of carbon dioxide ($pCO_2$, regulated by the lungs). 1. **Why Option B is Correct:** The standard physiological range for arterial bicarbonate is **22–28 mEq/L** (often simplified in exams to **20–24 mEq/L** or **24 mEq/L** as the mean). This concentration is necessary to maintain the 20:1 ratio of $HCO_3^-$ to $H_2CO_3$ required for a normal physiological pH. 2. **Why Other Options are Incorrect:** * **Option A (40-45 mEq/L):** This represents severe metabolic alkalosis or significant renal compensation for chronic respiratory acidosis. * **Options C & D (5-15 mEq/L):** These values indicate severe metabolic acidosis (e.g., Diabetic Ketoacidosis or Renal Failure), which would lead to life-threatening acidemia. **NEET-PG High-Yield Pearls:** * **Venous vs. Arterial:** Venous $HCO_3^-$ is slightly higher than arterial $HCO_3^-$ (by ~2–4 mEq/L) because CO2 is picked up from tissues and converted to bicarbonate via carbonic anhydrase in RBCs. * **Anion Gap:** Bicarbonate is the primary "measured anion" used to calculate the Anion Gap ($AG = Na^+ - [Cl^- + HCO_3^-]$). * **Base Excess:** A deviation in $HCO_3^-$ from the normal range is reflected in the 'Base Excess' or 'Base Deficit' on an ABG report. * **The 20:1 Rule:** At a pH of 7.4, the ratio of $HCO_3^-$ to dissolved $CO_2$ must be exactly 20:1.

Practice by Chapter

Acid-Base Chemistry

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Respiratory Regulation of Acid-Base Balance

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Renal Regulation of Acid-Base Balance

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Bicarbonate Buffer System

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Non-Bicarbonate Buffer Systems

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Respiratory Acidosis and Alkalosis

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Metabolic Acidosis and Alkalosis

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Mixed Acid-Base Disorders

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Compensatory Mechanisms

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Clinical Assessment of Acid-Base Status

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