Acid-Base Balance Practice Questions

Q: Which of the following conditions is NOT associated with an increased anion-gap metabolic acidosis?

COPD. **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. An increased anion gap metabolic acidosis (HAGMA) occurs when unmeasured anions (like lactate, ketones, or exogenous toxins) accumulate in the blood, consuming bicarbonate. **Why COPD is the correct answer:** COPD (Chronic Obstructive Pulmonary Disease) is a condition of impaired gas exchange leading to the retention of $CO_2$. This results in **Respiratory Acidosis**, not metabolic acidosis. In chronic COPD, the kidneys compensate by *increasing* bicarbonate reabsorption, which is the opposite of what occurs in metabolic acidosis. **Analysis of Incorrect Options:** * **Shock (Option A):** Leads to tissue hypoperfusion and anaerobic metabolism, causing **Lactic Acidosis**. Lactate is an unmeasured anion that increases the AG. * **Ingestion of Antifreeze (Option B):** Antifreeze contains **Ethylene Glycol**, which is metabolized into toxic acids (glycolic and oxalic acid). These unmeasured anions cause a significant HAGMA. * **Diabetic Ketoacidosis (Option C):** Insulin deficiency leads to the production of **acetoacetate and beta-hydroxybutyrate**. These ketoacids increase the AG. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic for HAGMA:** "MUDPILES" (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). 2. **Normal Anion Gap Acidosis (NAGMA):** Primarily caused by **Diarrhea** or **Renal Tubular Acidosis (RTA)**. Here, the loss of $HCO_3^-$ is replaced by $Cl^-$, keeping the gap normal (Hyperchloremic acidosis). 3. **Winter’s Formula:** Used to calculate expected $pCO_2$ compensation in metabolic acidosis: $pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$.

Q: Which of the following causes metabolic alkalosis?

Hyperaldosteronism. **Explanation:** **Correct Option: C. Hyperaldosteronism** Aldosterone acts on the principal cells of the collecting duct to reabsorb sodium and secrete potassium. Crucially, it also stimulates the **$\alpha$-intercalated cells** to secrete hydrogen ions ($H^+$) via the $H^+$-ATPase pump. In hyperaldosteronism (e.g., Conn’s Syndrome), excessive $H^+$ secretion into the urine leads to a gain of bicarbonate in the blood, resulting in **metabolic alkalosis**. This is typically accompanied by hypokalemia and hypertension. **Incorrect Options:** * **A. Diarrhea:** Causes loss of bicarbonate-rich intestinal secretions, leading to **Normal Anion Gap Metabolic Acidosis (NAGMA)**. * **B. Chronic Renal Failure:** The kidneys fail to excrete fixed acids (phosphates/sulfates) and cannot regenerate sufficient bicarbonate, leading to **High Anion Gap Metabolic Acidosis (HAGMA)**. * **D. Salicylate Poisoning:** Classically causes a mixed acid-base disorder: **Respiratory Alkalosis** (due to direct stimulation of the medullary respiratory center) and **High Anion Gap Metabolic Acidosis** (due to interference with the Krebs cycle and accumulation of organic acids). **High-Yield Clinical Pearls for NEET-PG:** * **Metabolic Alkalosis "Mnemonic":** Remember **"Vomiting & Volume depletion"** and **"Hyperaldosteronism"** as the primary causes. * **Paradoxical Aciduria:** In states of metabolic alkalosis with hypokalemia (like hyperaldosteronism), the kidney excretes $H^+$ instead of $K^+$ to conserve potassium, making the urine acidic despite systemic alkalosis. * **Salicylate Poisoning:** In children, metabolic acidosis often dominates; in adults, a mixed picture is more common.

Q: A patient presents with pO2 of 85 mmHg, pCO2 of 50 mmHg, pH of 7.2, and HCO3- of 36 mmol/L. What is the acid-base condition?

Respiratory acidosis with compensatory metabolic alkalosis. ### Explanation To solve acid-base questions, follow a systematic three-step approach: **1. Analyze the pH:** The normal pH range is 7.35–7.45. A pH of **7.20** indicates **acidemia**. **2. Identify the Primary Cause:** * **pCO2:** The normal range is 35–45 mmHg. Here, pCO2 is **50 mmHg** (elevated). High CO2 causes acidosis (Respiratory Acidosis). * **HCO3-:** The normal range is 22–28 mmol/L. Here, HCO3- is **36 mmol/L** (elevated). High bicarbonate causes alkalosis. Since the pH is acidic, the primary driver must be the elevated pCO2. Thus, the primary condition is **Respiratory Acidosis**. **3. Determine Compensation:** The body attempts to neutralize the acidosis by increasing HCO3- (Metabolic Alkalosis). Since the HCO3- is significantly elevated (36 mmol/L) and the pH is moving back toward normal (but not yet fully normal), it is **Respiratory acidosis with compensatory metabolic alkalosis**. #### Analysis of Incorrect Options: * **Option B:** In "uncompensated" acidosis, the HCO3- would remain within the normal range (22–28 mmol/L). * **Option C:** Respiratory alkalosis would require a high pH (>7.45) and a low pCO2 (<35 mmHg). * **Option D:** Metabolic acidosis would involve a low HCO3- ( 7.40, it is alkalosis. * **Renal Compensation:** Unlike respiratory compensation (which takes minutes), renal compensation (altering HCO3- reabsorption) takes **24–72 hours** to reach maximal effect.

Q: A 41-year-old woman treated with acetazolamide develops an arterial pH of 7.34, an arterial PCO2 of 29 mmHg, and a plasma HCO3- of 15 mEq/L. Which of the following abnormalities has this woman most likely developed?

Metabolic acidosis. ### **Explanation** **1. Analysis of the Correct Answer (Metabolic Acidosis):** To determine the acid-base status, follow a systematic approach: * **pH (7.34):** The normal range is 7.35–7.45. A pH of 7.34 indicates **acidemia**. * **Primary Change:** Look at the Bicarbonate ($HCO_3^-$) and $PCO_2$. The $HCO_3^-$ is low (15 mEq/L; normal: 22–26), which correlates with the acidic pH. This confirms a **primary metabolic acidosis**. * **Compensation:** The $PCO_2$ is low (29 mmHg; normal: 35–45). Using **Winters’ Formula** ($Expected\ PCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2$), the expected $PCO_2$ is $1.5(15) + 8 = 30.5 \pm 2$ (range: 28.5–32.5). Since the measured $PCO_2$ (29) falls within this range, it is a **simple metabolic acidosis with appropriate respiratory compensation.** **Mechanism of Acetazolamide:** It is a carbonic anhydrase inhibitor that blocks $HCO_3^-$ reabsorption in the proximal convoluted tubule, leading to bicarbonate loss in the urine (bicarbonaturia), resulting in **Normal Anion Gap Metabolic Acidosis (NAGMA).** **2. Why Other Options are Incorrect:** * **Metabolic Alkalosis:** This would present with a pH >7.45 and an elevated $HCO_3^-$. * **Mixed Acidosis:** This would occur if the $PCO_2$ was higher than the expected compensated range (e.g., >33 mmHg), indicating a concurrent respiratory acidosis. * **Mixed Alkalosis:** This would occur if the $PCO_2$ was significantly lower than the expected range (e.g., <28 mmHg), indicating a concurrent respiratory alkalosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Acetazolamide** is a classic cause of **NAGMA** (along with diarrhea and RTA). * **Winters' Formula** is essential for identifying mixed disorders in metabolic acidosis. * **Rule of Thumb:** In simple acid-base disorders, pH and $PCO_2$ move in the **same direction** in metabolic issues (both down in acidosis) and **opposite directions** in respiratory issues.

Q: Which is the slowest acting buffer system in the human body?

Renal regulation of bicarbonate and hydrogen ions. The human body maintains acid-base balance through three primary lines of defense, which differ significantly in their speed and capacity. ### **Explanation of the Correct Answer** **D. Renal regulation of bicarbonate and hydrogen ions** is the slowest but most powerful buffer system. While chemical and respiratory systems respond in seconds to minutes, the kidneys require **several hours to 3–5 days** to reach maximal effectiveness. The renal system regulates pH by excreting hydrogen ions ($H^+$), reabsorbing filtered bicarbonate ($HCO_3^-$), and generating new bicarbonate. Because this involves physical transport of ions and metabolic synthesis (like ammonia production), it is inherently the most time-consuming mechanism. ### **Analysis of Incorrect Options** * **A & C (Immediate chemical buffers):** These represent the **first line of defense**. Systems like bicarbonate, phosphate, and proteins (e.g., hemoglobin) act **instantaneously** (within seconds) to minimize pH changes. They are the fastest but have limited capacity. * **B (Respiratory regulation):** This is the **second line of defense**. By altering the rate and depth of ventilation to eliminate or retain $CO_2$, the lungs can adjust pH within **1 to 15 minutes**. It is significantly faster than the renal system. ### **NEET-PG High-Yield Pearls** * **Hierarchy of Speed:** Chemical Buffers (Seconds) > Respiratory System (Minutes) > Renal System (Days). * **Hierarchy of Power:** Renal > Respiratory > Chemical. The kidneys are the only system that can eliminate fixed acids (like lactic acid or phosphoric acid) from the body. * **The Bicarbonate Buffer System** is the most important **extracellular** buffer, while **Proteins and Phosphates** are the primary **intracellular** buffers. * **Ammonia ($NH_3$)** is the most important urinary buffer for the excretion of $H^+$ during chronic acidosis.

Q: Most severe degree of alkalosis occurs in obstruction of which anatomical region?

Pylorus. **Explanation:** The correct answer is **Pylorus (Option B)**. The severity of alkalosis in gastrointestinal obstruction depends on the loss of gastric acid (HCl). **Why Pylorus is Correct:** Obstruction at the pylorus (e.g., Gastric Outflow Obstruction or Congenital Hypertrophic Pyloric Stenosis) leads to persistent, non-bilious vomiting. Gastric juice is rich in **Hydrogen (H⁺)** and **Chloride (Cl⁻)** ions. When these are lost, the body experiences: 1. **Metabolic Alkalosis:** Due to the loss of H⁺ ions. 2. **Hypochloremia:** Due to the loss of Cl⁻ ions. 3. **Hypokalemia:** As the kidneys attempt to conserve H⁺ at the expense of K⁺. This classic triad results in **Hypochloremic Hypokalemic Metabolic Alkalosis**, the most severe form of alkalosis seen in GI obstructions. **Why Other Options are Incorrect:** * **Cardiac End (A):** Obstruction here (e.g., Achalasia) prevents food from entering the stomach. While it causes regurgitation, it does not involve the loss of secreted gastric acid, thus not causing significant alkalosis. * **Ileocaecal Region (C) & Colon (D):** Obstructions distal to the stomach (lower GI) result in the loss of alkaline intestinal secretions (rich in bicarbonate). This typically leads to **Metabolic Acidosis**, not alkalosis. **High-Yield Clinical Pearls for NEET-PG:** * **Paradoxical Aciduria:** In severe pyloric obstruction, despite systemic alkalosis, the urine becomes acidic. This occurs because the kidney prioritizes volume expansion (reabsorbing Na⁺) over pH balance, excreting H⁺ ions to save Na⁺ when K⁺ stores are depleted. * **Treatment of Choice:** Intravenous **0.9% Normal Saline** (to replace Cl⁻ and volume) with Potassium supplementation. Avoid glucose-only fluids initially as they can worsen hypokalemia.

Q: In metabolic acidosis caused by diabetic ketoacidosis, which of the following would be greater than normal?

Anion gap. In metabolic acidosis, specifically Diabetic Ketoacidosis (DKA), the primary pathology is the accumulation of fixed organic acids (acetoacetate and β-hydroxybutyrate). **1. Why the Anion Gap is increased:** The Anion Gap (AG) is calculated as: $[Na^+] - ([Cl^-] + [HCO_3^-])$. In DKA, the ketoacids dissociate into hydrogen ions ($H^+$) and unmeasured anions (ketoanions). The $H^+$ ions are buffered by $HCO_3^-$, causing the bicarbonate level to drop. However, the chloride level remains relatively stable. Because the $HCO_3^-$ is replaced by unmeasured ketoanions rather than chloride, the gap between measured cations and measured anions increases. This makes DKA a classic example of **High Anion Gap Metabolic Acidosis (HAGMA).** **2. Why other options are incorrect:** * **Option A (Plasma $HCO_3^-$):** In any metabolic acidosis, bicarbonate is consumed as it acts as the primary buffer for excess $H^+$ ions. Therefore, $HCO_3^-$ levels will be **lower** than normal. * **Option C (Arterial $pCO_2$):** The body attempts to compensate for metabolic acidosis through the respiratory system. The low pH stimulates chemoreceptors to increase the rate and depth of breathing (**Kussmaul respiration**), which "blows off" $CO_2$. Consequently, $pCO_2$ will be **lower** than normal (respiratory compensation). **Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Winter’s Formula:** Used to calculate expected $pCO_2$ compensation: $Expected\ pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$. * **Normal Anion Gap:** 8–12 mEq/L. DKA typically presents with an AG > 12.

Q: A 65-year-old man with mild heart failure is treated with a loop diuretic. A few days later, the man complains of muscle weakness. Laboratory results show: PaCO2: 48 mm Hg, pH: 7.49, Plasma HCO3-: 35 mEq/L. Which of the following is most likely decreased in this man?

Plasma potassium. **Explanation:** The patient is presenting with **Metabolic Alkalosis** (pH 7.49, high $HCO_3^-$) and compensatory respiratory acidosis (high $PaCO_2$), secondary to **loop diuretic** use. **Why Plasma Potassium is decreased:** Loop diuretics (e.g., Furosemide) inhibit the $Na^+/K^+/2Cl^-$ cotransporter in the Thick Ascending Limb. This leads to: 1. **Increased distal delivery of $Na^+$:** This stimulates the $Na^+/K^+$ exchange in the collecting duct, causing excessive $K^+$ secretion into the urine. 2. **Volume Depletion:** This activates the **Renin-Angiotensin-Aldosterone System (RAAS)**. Aldosterone further increases $K^+$ and $H^+$ secretion in the distal tubule. 3. **Contraction Alkalosis:** Loss of fluid leads to a relative increase in $HCO_3^-$ concentration. The resulting **hypokalemia** causes the muscle weakness described in the clinical vignette. **Analysis of Incorrect Options:** * **A & D (Plasma Angiotensin/Renin):** These would be **increased**, not decreased. Diuretic-induced volume depletion triggers the RAAS to maintain blood pressure and sodium levels. * **C (Potassium excretion):** This would be **increased**. The mechanism of loop diuretics inherently forces more potassium into the urine through both flow-dependent and aldosterone-mediated mechanisms. **NEET-PG High-Yield Pearls:** * **Loop and Thiazide diuretics** both cause **Hypokalemic Metabolic Alkalosis**. * **Acetazolamide** (Carbonic anhydrase inhibitor) causes **Hyperchloremic Metabolic Acidosis**. * **Spironolactone** (K-sparing) causes **Hyperkalemic Metabolic Acidosis**. * **Mnemonic:** Loop and Thiazides "lose" $K^+$ and $H^+$, making the blood "basic" (Alkalosis).

Q: A patient's arterial blood gas results show a pH of 7.28 and a PaCO2 of 70. Which of the following acid-base conditions is consistent with these findings?

Respiratory acidosis. ### **Explanation** To solve any Acid-Base question, follow a systematic two-step approach: **1. Determine the pH status:** The normal arterial pH range is **7.35 – 7.45**. * In this case, the pH is **7.28**, which is < 7.35. This indicates **Acidosis**. **2. Determine the primary cause (Respiratory vs. Metabolic):** * **Respiratory:** Look at the PaCO2 (Normal: 35–45 mmHg). CO2 acts as an acid in the body. An increase in PaCO2 (Hypercapnia) causes respiratory acidosis. * **Metabolic:** Look at the HCO3- (Normal: 22–26 mEq/L). A decrease in HCO3- causes metabolic acidosis. * In this patient, the **PaCO2 is 70 mmHg**, which is significantly elevated (> 45 mmHg). This high level of CO2 is the primary driver of the low pH. **Conclusion:** The combination of a low pH and high PaCO2 confirms **Respiratory Acidosis**. --- ### **Why the other options are incorrect:** * **Metabolic Acidosis:** While the pH would be low ( 7.45) due to an increase in HCO3-. * **Respiratory Alkalosis:** The pH would be high (> 7.45) due to a decrease in PaCO2 (hypocapnia), often seen in hyperventilation. --- ### **High-Yield NEET-PG Pearls:** * **ROME Mnemonic:** **R**espiratory **O**pposite (pH ↓, CO2 ↑ or pH ↑, CO2 ↓), **M**etabolic **E**qual (pH ↓, HCO3- ↓ or pH ↑, HCO3- ↑). * **Common Causes of Respiratory Acidosis:** Hypoventilation, COPD, Opioid overdose, and Myasthenia Gravis. * **Compensation:** In respiratory acidosis, the kidneys compensate by retaining HCO3-. This takes 24–48 hours to occur (Acute vs. Chronic).

Q: Which of the following can cause metabolic alkalosis?

Furosemide. **Explanation:** **Correct Answer: A. Furosemide** Furosemide is a loop diuretic that inhibits the $Na^+-K^+-2Cl^-$ symporter in the thick ascending limb of Henle. It causes metabolic alkalosis through three primary mechanisms: 1. **Contraction Alkalosis:** Loss of isotonic fluid (sodium and water) leads to extracellular fluid volume depletion, concentrating the existing bicarbonate. 2. **Increased Distal Delivery:** Increased $Na^+$ delivery to the distal tubule stimulates $Na^+$ reabsorption in exchange for $H^+$ and $K^+$ ions (via aldosterone activation), leading to net acid loss. 3. **Hypokalemia:** Diuretic-induced potassium loss causes a transcellular shift where $K^+$ moves out of cells and $H^+$ moves in, further raising extracellular pH. **Why the other options are incorrect:** * **B. Addison’s Disease:** This is primary adrenal insufficiency (low aldosterone). Aldosterone deficiency leads to decreased $H^+$ secretion in the distal tubule, resulting in **Normal Anion Gap Metabolic Acidosis**, not alkalosis. * **C. Hyperkalemia:** High serum potassium causes $K^+$ to enter cells in exchange for $H^+$ moving into the extracellular fluid. This intracellular buffering results in **Metabolic Acidosis**. * **D. Hyponatremia:** While often associated with various acid-base disturbances, hyponatremia itself is an electrolyte imbalance and not a direct cause of metabolic alkalosis. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Metabolic Alkalosis:** "CLEVER" (Contraction, Licorice, Endocrine [Conn’s/Cushing’s], Vomiting, Excess Alkali, Renal [Bartter/Gitelman/Diuretics]). * **Chloride Status:** Diuretic-induced alkalosis is typically **Saline-Responsive** (Urinary $Cl^-$ < 20 mEq/L). * **Bartter vs. Gitelman:** Bartter syndrome mimics Loop diuretics (Furosemide), while Gitelman syndrome mimics Thiazide diuretics; both present with metabolic alkalosis.

Question 1

Which of the following conditions is NOT associated with an increased anion-gap metabolic acidosis?

Accepted Answer

COPD

Answer

Shock

Answer

Ingestion of antifreeze

Answer

Diabetic ketoacidosis

Question 2

Which of the following causes metabolic alkalosis?

Accepted Answer

Hyperaldosteronism

Answer

Diarrhea

Answer

Chronic renal failure

Answer

Salicylate poisoning

Question 3

A patient presents with pO2 of 85 mmHg, pCO2 of 50 mmHg, pH of 7.2, and HCO3- of 36 mmol/L. What is the acid-base condition?

Accepted Answer

Respiratory acidosis with compensatory metabolic alkalosis

Answer

Uncompensated respiratory acidosis

Answer

Respiratory alkalosis with compensatory metabolic acidosis

Answer

Respiratory acidosis with compensatory metabolic acidosis

Question 4

A 41-year-old woman treated with acetazolamide develops an arterial pH of 7.34, an arterial PCO2 of 29 mmHg, and a plasma HCO3- of 15 mEq/L. Which of the following abnormalities has this woman most likely developed?

Accepted Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Mixed acidosis

Answer

Mixed alkalosis

Question 5

Which is the slowest acting buffer system in the human body?

Accepted Answer

Renal regulation of bicarbonate and hydrogen ions

Answer

Carbon dioxide/bicarbonate buffer system

Answer

Respiratory regulation of CO2

Answer

Immediate chemical buffers (e.g., bicarbonate, phosphate, proteins)

Question 6

Most severe degree of alkalosis occurs in obstruction of which anatomical region?

Accepted Answer

Pylorus

Answer

Cardiac end

Answer

Ileocaecal region

Answer

Colon

Question 7

In metabolic acidosis caused by diabetic ketoacidosis, which of the following would be greater than normal?

Accepted Answer

Anion gap

Answer

Concentration of plasma HCO3-

Answer

Arterial pCO2

Answer

All of the above

Question 8

A 65-year-old man with mild heart failure is treated with a loop diuretic. A few days later, the man complains of muscle weakness. Laboratory results show: PaCO2: 48 mm Hg, pH: 7.49, Plasma HCO3-: 35 mEq/L. Which of the following is most likely decreased in this man?

Accepted Answer

Plasma potassium

Answer

Plasma angiotensin

Answer

Potassium excretion

Answer

Renin secretion

Question 9

A patient's arterial blood gas results show a pH of 7.28 and a PaCO2 of 70. Which of the following acid-base conditions is consistent with these findings?

Accepted Answer

Respiratory acidosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory alkalosis

Question 10

Which of the following can cause metabolic alkalosis?

Accepted Answer

Furosemide

Answer

Addison's disease

Answer

Hyperkalemia

Answer

Hyponatremia

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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