Acid-Base Balance — MCQs

Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 61: Which of the following conditions is associated with paradoxical aciduria?

A. Intestinal obstruction
B. Pyloric obstruction (Correct Answer)
C. Enterocutaneous fistula
D. Vesicovaginal fistula

Explanation: **Explanation:** **Paradoxical aciduria** refers to the excretion of acidic urine in the presence of systemic metabolic alkalosis. This phenomenon is classically seen in **Pyloric Obstruction** (e.g., Congenital Hypertrophic Pyloric Stenosis). **Pathophysiology of Pyloric Obstruction:** 1. **Metabolic Alkalosis:** Persistent vomiting of gastric juice leads to a loss of $H^+$ and $Cl^-$, resulting in **hypochloremic hypokalemic metabolic alkalosis**. 2. **Volume Depletion:** Loss of fluid triggers the Renin-Angiotensin-Aldosterone System (RAAS). Aldosterone acts on the distal tubule to reabsorb $Na^+$ and water. 3. **The "Paradox":** To conserve $Na^+$, the kidney must excrete a positive ion. Initially, it excretes $K^+$. However, as the patient becomes severely **hypokalemic**, the kidney runs out of $K^+$ to exchange for $Na^+$. Consequently, it begins secreting **$H^+$ ions** into the urine instead. Despite the body being in a state of alkalosis, the urine becomes acidic to maintain sodium levels. **Analysis of Incorrect Options:** * **Intestinal obstruction:** Usually leads to the loss of alkaline succus entericus, potentially causing metabolic acidosis. * **Enterocutaneous/Vesicovaginal fistula:** These involve the loss of bicarbonate-rich intestinal or alkaline fluids, typically resulting in normal anion gap metabolic acidosis, not alkalosis. **High-Yield Clinical Pearls for NEET-PG:** * **Electrolyte Triad in Pyloric Stenosis:** Hypochloremia, Hypokalemia, and Metabolic Alkalosis. * **Key Trigger:** The primary driver for paradoxical aciduria is **volume depletion** (RAAS activation) coupled with **severe hypokalemia**. * **Treatment:** The definitive initial management is resuscitation with **0.9% Normal Saline** (to correct volume and chloride) and **Potassium** supplementation; once $K^+$ is replaced, the kidney stops secreting $H^+$, and the paradox resolves.

Question 62: Features seen in a patient with chronic vomiting include?

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

Explanation: **Explanation:** Chronic vomiting leads to a classic metabolic derangement known as **Metabolic Alkalosis with Paradoxical Aciduria**. **Why Hypochloremia is the primary feature:** Gastric juice is rich in Hydrochloric acid (HCl). Persistent vomiting results in the direct loss of chloride ions ($Cl^-$) and hydrogen ions ($H^+$). As chloride levels fall, the kidneys attempt to maintain electrical neutrality by reabsorbing more bicarbonate ($HCO_3^-$) in the proximal tubule, leading to **Hypochloremic Metabolic Alkalosis**. This is the hallmark electrolyte abnormality in gastric outlet obstruction or chronic vomiting. **Analysis of Options:** * **B. Hypochloremia (Correct):** Direct loss of $Cl^-$ in gastric secretions. * **C. Metabolic Alkalosis:** While this occurs, the question asks for "features" (plural), but in many competitive exams, if forced to choose the most specific electrolyte deficit directly lost from the stomach, Hypochloremia is prioritized. However, in clinical practice, **B, C, and D are all typically seen.** * **D. Hypokalemia:** This occurs due to two reasons: 1) Direct loss in vomitus (minor) and 2) Renal compensation where the body exchanges $K^+$ for $H^+$ to conserve acid, and aldosterone-mediated $K^+$ loss due to volume depletion. * **A. Hyponatremia:** Sodium is lost, but the body often compensates via aldosterone, making it less characteristic than the chloride/potassium shifts. **High-Yield NEET-PG Pearls:** 1. **Paradoxical Aciduria:** In severe vomiting, despite systemic alkalosis, the urine is acidic. This happens because the kidney prioritizes volume (sodium reabsorption) over pH, excreting $H^+$ ions to save $Na^+$ when $K^+$ is depleted. 2. **Treatment of choice:** Isotonic Saline (0.9% NaCl). It corrects volume depletion and provides chloride, allowing the kidney to excrete the excess bicarbonate. 3. **Formula:** Loss of HCl → Hypochloremia → Increased $HCO_3^-$ reabsorption → Metabolic Alkalosis.

Question 63: Which of the following is NOT a cause of metabolic alkalosis?

A. Mineralocorticoid deficiency (Correct Answer)
B. Bartter's syndrome
C. Thiazide diuretic therapy
D. Recurrent vomiting

Explanation: **Explanation:** **1. Why Mineralocorticoid deficiency is the correct answer:** Mineralocorticoids (primarily Aldosterone) act on the distal tubule and collecting duct to reabsorb Na+ and water in exchange for the secretion of **K+ and H+ ions**. In **Mineralocorticoid deficiency** (e.g., Addison’s disease), there is a failure to secrete H+ ions and K+. This leads to the retention of H+ ions, resulting in **Normal Anion Gap Metabolic Acidosis** (specifically Type 4 Renal Tubular Acidosis) and hyperkalemia. Therefore, it causes acidosis, not alkalosis. **2. Why the other options are incorrect:** * **Bartter's Syndrome:** This is a genetic defect in the thick ascending limb (NKCC2 transporter), mimicking chronic loop diuretic use. It leads to increased distal delivery of Na+, causing secondary hyperaldosteronism, which promotes H+ secretion and results in **Metabolic Alkalosis**. * **Thiazide Diuretic Therapy:** Thiazides inhibit the Na-Cl symporter in the distal tubule. The resulting volume depletion activates the Renin-Angiotensin-Aldosterone System (RAAS). Increased aldosterone promotes H+ loss in the urine, leading to **"Contraction Alkalosis."** * **Recurrent Vomiting:** Gastric juice is rich in HCl. Loss of stomach acid directly removes H+ ions from the body. Additionally, the loss of fluid leads to volume depletion and activation of aldosterone, further worsening the **Metabolic Alkalosis**. **High-Yield Clinical Pearls for NEET-PG:** * **Aldosterone Excess** (Conn’s Syndrome, Cushing’s) = Metabolic Alkalosis + Hypokalemia. * **Aldosterone Deficiency** (Addison’s) = Metabolic Acidosis + Hyperkalemia. * **Saline-Responsive Alkalosis:** Vomiting and Diuretics (Urine Cl- < 10-20 mEq/L). * **Saline-Resistant Alkalosis:** Bartter’s, Gitelman’s, and Mineralocorticoid excess (Urine Cl- > 20 mEq/L).

Question 64: Which of the following causes normal anion gap metabolic acidosis?

A. Cholera (Correct Answer)
B. Starvation
C. Ethylene glycol poisoning
D. Lactic acidosis

Explanation: **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. **1. Why Cholera is Correct:** Cholera causes profuse, watery diarrhea. Intestinal secretions are rich in bicarbonate ($HCO_3^-$). The direct loss of bicarbonate leads to a **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as hyperchloremic metabolic acidosis. To maintain electroneutrality, the kidneys retain Chloride ($Cl^-$) to replace the lost bicarbonate, keeping the anion gap within the normal range (8–12 mEq/L). **2. Why the Incorrect Options are Wrong:** All other options (B, C, and D) cause **High Anion Gap Metabolic Acidosis (HAGMA)**. In these conditions, an unmeasured acid anion is added to the blood, which "consumes" bicarbonate without a corresponding increase in chloride. * **Starvation:** Leads to the accumulation of ketoacids (acetoacetate and beta-hydroxybutyrate). * **Ethylene Glycol:** Metabolized into glycolic and oxalic acids. * **Lactic Acidosis:** Occurs due to tissue hypoxia, leading to the accumulation of lactate. **Clinical Pearls for NEET-PG:** * **Mnemonic for NAGMA (USED CARP):** **U**reterosigmoidostomy, **S**aline infusion, **E**ndocrine (Addison’s), **D**iarrhea (Cholera), **C**arbonic anhydrase inhibitors (Acetazolamide), **A**mmonium chloride, **R**enal tubular acidosis (RTA), **P**ancreatic fistula. * **Mnemonic for HAGMA (MUDPILES):** **M**ethanol, **U**remia, **D**KA/Starvation, **P**araldehyde, **I**NH/Iron, **L**actic acidosis, **E**thylene glycol, **S**alicylates. * **Key Distinction:** If the question mentions GI loss (diarrhea) or Renal loss (RTA), always think **NAGMA**. If it mentions ingestion of toxins or endogenous acid production, think **HAGMA**.

Question 65: A patient has the following blood gases: PaCO2- 25, pH- 7.50, HCO3-- 19. Which of the following could not be the cause of this condition?

A. Anxiety attack
B. PaO2/FiO2 ratio <100 (Correct Answer)
C. Hysteria
D. Aspirin toxicity

Explanation: ### **Explanation** **1. Analysis of Blood Gas Parameters:** * **pH 7.50:** Indicates **Alkalosis** (Normal: 7.35–7.45). * **PaCO2 25 mmHg:** Indicates **Respiratory Alkalosis** (Normal: 35–45 mmHg). * **HCO3⁻ 19 mEq/L:** Indicates a compensatory decrease (Normal: 22–26 mEq/L). The primary disturbance is **Respiratory Alkalosis**. **2. Why Option B is the Correct Answer:** A **PaO2/FiO2 ratio <100** is a hallmark of severe **Acute Respiratory Distress Syndrome (ARDS)**. In severe ARDS, there is a significant "shunt" and loss of lung compliance, leading to refractory hypoxemia and eventually **Respiratory Acidosis** (due to respiratory failure and CO2 retention) or **Metabolic Acidosis** (due to lactic acid from tissue hypoxia). It does not typically present as a simple, compensated respiratory alkalosis in its severe stage. **3. Analysis of Incorrect Options:** * **Anxiety Attack & Hysteria (Options A & C):** These are classic causes of hyperventilation. Excessive "blowing off" of CO2 leads to an increase in pH, resulting in acute respiratory alkalosis. * **Aspirin Toxicity (Option D):** Salicylates directly stimulate the medullary respiratory center, causing hyperventilation and **Respiratory Alkalosis** as the *earliest* acid-base disturbance (often followed by a mixed metabolic acidosis in later stages). ### **High-Yield Clinical Pearls for NEET-PG:** * **Rule of Thumb:** In Respiratory Alkalosis, for every 10 mmHg drop in PaCO2, the pH rises by 0.08 (Acute) or 0.03 (Chronic). * **Salicylate Poisoning:** The most common mixed acid-base disorder in adults is **Respiratory Alkalosis + Metabolic Acidosis**. * **P/F Ratio Categories (Berlin Criteria):** * Mild ARDS: 200–300 * Moderate ARDS: 100–200 * Severe ARDS: <100

Question 66: What condition is characterized by a primary increase in CO2 in the body?

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

Explanation: ### Explanation **1. Why Respiratory Acidosis is Correct:** Respiratory acidosis is defined by a **primary increase in the partial pressure of carbon dioxide ($PCO_2$)** in the arterial blood (hypercapnia). This occurs due to **alveolar hypoventilation**, where the lungs fail to eliminate $CO_2$ as fast as it is produced by tissues. According to the Henderson-Hasselbalch equation, an increase in $CO_2$ leads to an increase in $H^+$ concentration, thereby lowering the pH ($<7.35$). Common causes include COPD, respiratory muscle paralysis, or depression of the respiratory center (e.g., opioid overdose). **2. Why the Other Options are Incorrect:** * **Respiratory Alkalosis:** Characterized by a primary **decrease** in $PCO_2$ due to alveolar hyperventilation (e.g., high altitude, anxiety). * **Metabolic Acidosis:** Characterized by a primary **decrease in bicarbonate ($HCO_3^-$)** or an increase in non-volatile acids (e.g., Diabetic Ketoacidosis). * **Metabolic Alkalosis:** Characterized by a primary **increase in bicarbonate ($HCO_3^-$)** (e.g., persistent vomiting or diuretic use). **3. NEET-PG High-Yield Pearls:** * **The Golden Rule:** In primary respiratory disturbances, the pH and $PCO_2$ move in **opposite** directions (Respiratory Opposite). In metabolic disturbances, pH and $HCO_3^-$ move in the **same** direction (Metabolic Equal). * **Compensation:** In respiratory acidosis, the kidneys compensate by **reabsorbing $HCO_3^-$** and excreting $H^+$. * **Acute vs. Chronic:** For every 10 mmHg rise in $PCO_2$, the $HCO_3^-$ rises by **1 mEq/L** in acute respiratory acidosis and **3.5–4 mEq/L** in chronic respiratory acidosis.

Question 67: Sustained diarrhea can lead to which of the following acid-base disturbances?

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

Explanation: **Explanation:** **1. Why Metabolic Acidosis is Correct:** The gastrointestinal tract below the stomach is rich in **bicarbonate (HCO₃⁻)**, which is secreted by the pancreas and intestinal mucosa to neutralize gastric acid. In sustained diarrhea, there is a significant loss of these alkaline intestinal secretions. The loss of bicarbonate leads to a decrease in the blood's buffering capacity, resulting in a **Normal Anion Gap Metabolic Acidosis (NAGMA)**. To maintain electrical neutrality, the body compensates for the lost bicarbonate by retaining chloride ions, which is why this condition is also termed **hyperchloremic metabolic acidosis**. **2. Why the Other Options are Incorrect:** * **Metabolic Alkalosis:** This occurs with the loss of acid (H⁺), typically seen in **persistent vomiting** or nasogastric suction, where gastric HCl is lost. * **Respiratory Acidosis:** This is caused by **hypoventilation** leading to CO₂ retention (e.g., COPD, opioid overdose), not by GI losses. * **Respiratory Alkalosis:** This results from **hyperventilation** (e.g., anxiety, high altitude) leading to excessive "blowing off" of CO₂. **3. Clinical Pearls for NEET-PG:** * **Anion Gap Status:** Diarrhea is the most common cause of **Normal Anion Gap Metabolic Acidosis (NAGMA)**. Remember the mnemonic **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhea, Carbonic anhydrase inhibitors, Renal tubular acidosis, Pancreatic fistula). * **Potassium Status:** Diarrhea also leads to significant potassium loss, often resulting in **hypokalemia** alongside the acidosis. * **Compensation:** In metabolic acidosis, the lungs compensate via **Kussmaul breathing** (deep, rapid respirations) to decrease PaCO₂.

Question 68: All of the following cause high anion gap metabolic acidosis EXCEPT?

A. Lactic acidosis
B. Salicylate poisoning
C. Ethylene glycol poisoning
D. Ureterosigmoidostomy (Correct Answer)

Explanation: **Explanation:** Metabolic acidosis is classified into two categories based on the **Anion Gap (AG)**: High Anion Gap Metabolic Acidosis (HAGMA) and Normal Anion Gap Metabolic Acidosis (NAGMA). **1. Why Ureterosigmoidostomy is the correct answer:** Ureterosigmoidostomy causes **NAGMA** (Hyperchloremic metabolic acidosis). When ureters are diverted into the sigmoid colon, the intestinal mucosa is exposed to urine. The colon reabsorbs chloride ($Cl^-$) in exchange for bicarbonate ($HCO_3^-$) secretion into the bowel lumen. This loss of bicarbonate leads to acidosis, but because the chloride levels rise to compensate for the lost bicarbonate, the anion gap remains normal. **2. Why the other options are incorrect (HAGMA causes):** In HAGMA, an unmeasured organic acid accumulates, consuming bicarbonate without increasing chloride. * **Lactic Acidosis:** Accumulation of lactate (e.g., in shock or sepsis) increases the AG. * **Salicylate Poisoning:** Aspirin overdose leads to the accumulation of salicylic acid and interferes with the Krebs cycle, producing organic acids. * **Ethylene Glycol Poisoning:** Metabolism of this antifreeze agent produces glycolic and oxalic acids, leading to a high AG and often "envelope-shaped" calcium oxalate crystals in urine. **Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA:** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhea, Carbonic anhydrase inhibitors, Renal tubular acidosis, Pancreatic fistula). * **Key Distinction:** Diarrhea is the most common cause of NAGMA, while Lactic acidosis is the most common cause of HAGMA in clinical practice.

Question 69: An ABG analysis shows pH 7.2, PaCO2, and HCO3. Which of the following conditions does this represent?

A. Respiratory and metabolic acidosis (Correct Answer)
B. Respiratory acidosis
C. Compensated metabolic acidosis
D. Respiratory alkalosis

Explanation: To solve acid-base questions for NEET-PG, follow a systematic three-step approach: ### 1. Analysis of the Correct Answer * **Step 1 (pH):** The pH is **7.2**, which is significantly below the normal range (7.35–7.45). This confirms a state of **Acidosis**. * **Step 2 (Respiratory Component):** In respiratory acidosis, $PaCO_2$ is elevated (>45 mmHg). * **Step 3 (Metabolic Component):** In metabolic acidosis, $HCO_3^-$ is decreased (<22 mEq/L). * **The Logic:** When both the respiratory parameter ($PaCO_2 \uparrow$) and the metabolic parameter ($HCO_3^- \downarrow$) point toward acidosis, it is a **Mixed Acid-Base Disorder**. Specifically, it is a combined Respiratory and Metabolic Acidosis. This often occurs in clinical scenarios like cardio-pulmonary arrest or severe septic shock with respiratory failure. ### 2. Why Other Options are Incorrect * **B. Respiratory Acidosis:** This would typically show a high $PaCO_2$ with a *normal or slightly elevated* $HCO_3^-$ (due to renal compensation). * **C. Compensated Metabolic Acidosis:** In compensation, the pH returns toward the normal range (7.35–7.45). A pH of 7.2 indicates an **uncompensated** or mixed state. * **D. Respiratory Alkalosis:** This would present with a pH >7.45 and a low $PaCO_2$ (<35 mmHg), which contradicts the findings. ### 3. High-Yield Clinical Pearls for NEET-PG * **The "Same Direction" Rule:** In simple/primary disorders, $PaCO_2$ and $HCO_3^-$ move in the **same** direction. If they move in **opposite** directions (as in this case: $CO_2$ up, $HCO_3$ down), it is always a **Mixed Disorder**. * **Winter’s Formula:** Used to calculate expected $CO_2$ in metabolic acidosis: $Expected\ PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2$. * **Golden Rule:** Never diagnose "compensation" if the pH is still significantly abnormal; compensation rarely "over-corrects" the pH.

Question 70: A man is found comatose in his room, having taken an unknown number of sleeping pills. An arterial blood sample yields the following values: pH 7.10, HCO3 - 12 meq/liter, PaCO2 66 mm Hg. This patient's acid-base status is most accurately described as:

A. Uncompensated metabolic acidosis
B. Uncompensated respiratory acidosis
C. Mixed respiratory and metabolic acidosis (Correct Answer)
D. Respiratory acidosis with partial renal compensation

Explanation: ### Explanation To determine the acid-base status, we must analyze the pH, $PaCO_2$, and $HCO_3^-$ systematically: 1. **pH (7.10):** The normal range is 7.35–7.45. A pH of 7.10 indicates a severe **acidemia**. 2. **$PaCO_2$ (66 mm Hg):** The normal range is 35–45 mm Hg. An elevated $PaCO_2$ indicates **respiratory acidosis** (likely due to hypoventilation from the sleeping pill overdose). 3. **$HCO_3^-$ (12 mEq/L):** The normal range is 22–28 mEq/L. A low bicarbonate indicates **metabolic acidosis**. **Why C is correct:** In a simple acid-base disorder, the $PaCO_2$ and $HCO_3^-$ move in the **same direction** as the body attempts to compensate. Here, they are moving in **opposite directions** (high $CO_2$ and low $HCO_3^-$), both of which contribute to a lower pH. Since both the respiratory and metabolic components are promoting acidosis, this is a **mixed acid-base disorder**. **Why other options are wrong:** * **A & B:** These are incorrect because both systems (respiratory and metabolic) are abnormal and contributing to the acidemia. * **D:** In respiratory acidosis with renal compensation, the kidneys would *retain* bicarbonate to raise the pH. Here, the bicarbonate is low, worsening the acidosis rather than compensating for it. ### NEET-PG High-Yield Pearls * **The "Same Direction" Rule:** In all primary (simple) acid-base disturbances, $PaCO_2$ and $HCO_3^-$ always move in the same direction. If they move in opposite directions, it is a mixed disorder. * **Clinical Correlation:** Sleeping pill overdose (sedatives) causes respiratory depression, leading to $CO_2$ retention. The metabolic component (low $HCO_3^-$) in a comatose patient may result from lactic acidosis due to hypoxia or hypotension.

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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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