Acid-Base Balance Practice Questions

Q: Interpret the following lab values: pH 7.42, PaCO2 25 mmHg, HCO3- 18 mEq/L.

Respiratory alkalosis, fully compensated. ### Explanation To interpret acid-base disorders, follow a systematic three-step approach: 1. **Determine the pH status:** The pH is **7.42**. Since it falls within the normal range (7.35–7.45) but is on the alkaline side of the midpoint (7.40), it indicates a **fully compensated alkalosis**. 2. **Identify the primary cause:** * The **PaCO2 is 25 mmHg** (Normal: 40 mmHg). Low CO2 (hypocapnia) causes alkalosis. * The **HCO3- is 18 mEq/L** (Normal: 24 mEq/L). Low bicarbonate causes acidosis. Since the low PaCO2 matches the alkaline pH trend, the primary disturbance is **Respiratory Alkalosis**. 3. **Assess Compensation:** The kidneys have excreted HCO3- to bring the pH back into the normal range. Because the pH is now normal, it is **fully compensated**. #### Why the other options are incorrect: * **A & B (Partially compensated):** In partial compensation, the pH remains abnormal (below 7.35 or above 7.45). Here, the pH is 7.42 (normal). * **C (Respiratory acidosis):** In respiratory acidosis, the PaCO2 would be elevated (>45 mmHg) and the pH would be on the acidic side of normal ( 7.40 is compensated alkalosis. * **Renal Compensation Speed:** Metabolic compensation for respiratory disorders is slow (takes 2–5 days), whereas respiratory compensation for metabolic disorders is rapid (minutes to hours). * **Common Cause:** Hyperventilation (due to high altitude, anxiety, or pulmonary embolism) is the most frequent cause of respiratory alkalosis.

Q: All of the following are seen in persistent vomiting EXCEPT?

Decreased K+ in urine. In persistent vomiting, the body undergoes complex electrolyte and acid-base shifts. While it may seem counterintuitive, **urinary potassium levels actually increase**, making "Decreased K+ in urine" the correct exception. ### Why "Decreased K+ in urine" is the Correct Answer In persistent vomiting, there is a massive loss of HCl, leading to **Metabolic Alkalosis** (elevated blood pH). To compensate, the kidneys attempt to excrete excess bicarbonate ($HCO_3^-$). Because $HCO_3^-$ is a negatively charged ion, it must be excreted with a cation to maintain electrical neutrality. Since the body is trying to conserve Sodium ($Na^+$) due to dehydration (via Aldosterone), it excretes **Potassium ($K^+$)** instead. This phenomenon is known as **Paradoxical Kaliuresis**, leading to high levels of $K^+$ in the urine despite systemic hypokalemia. ### Why the other options are wrong: * **Hypokalemia (A):** Occurs due to three reasons: direct loss in vomitus, decreased oral intake, and primarily, the renal loss (kaliuresis) mentioned above. * **Elevated pH of blood (C) & Metabolic Alkalosis (D):** These are hallmarks of vomiting. Loss of gastric $H^+$ ions directly increases blood $HCO_3^-$ levels, raising the pH. ### NEET-PG High-Yield Pearls: * **Paradoxical Aciduria:** In severe, prolonged vomiting, the kidney eventually prioritizes $Na^+$ reabsorption over $H^+$ excretion to maintain blood volume. It begins excreting $H^+$ instead of $K^+$, leading to acidic urine despite systemic alkalosis. * **The "Vomiting Triad":** Hypokalemic, Hypochloremic, Metabolic Alkalosis. * **Aldosterone's Role:** Dehydration triggers the Renin-Angiotensin-Aldosterone System (RAAS). Aldosterone acts on the distal tubule to reabsorb $Na^+$ and excrete $K^+$ and $H^+$, further worsening the alkalosis and hypokalemia.

Q: A patient presents with a pH of 7.2, PO2 of 50 mm Hg, and HCO3– of 13 mEq/L. These values are most consistent with which of the following acid-base disturbances?

Mixed respiratory and metabolic acidosis. ### Explanation To solve acid-base problems, follow a systematic approach: **1. Analyze the pH:** The patient’s pH is **7.2** (Normal: 7.35–7.45). This indicates **acidemia**. **2. Identify the Metabolic Component (HCO3–):** The HCO3– is **13 mEq/L** (Normal: 22–26 mEq/L). A low bicarbonate level in the presence of acidemia signifies **metabolic acidosis**. **3. Identify the Respiratory Component (PCO2/PO2):** While PCO2 is the primary indicator for respiratory balance, we can infer the status from the clinical picture. In pure metabolic acidosis, the body should compensate by hyperventilating to "blow off" CO2 (Winter’s Formula). However, this patient has a **PO2 of 50 mm Hg** (Normal: 80–100 mm Hg), indicating significant **hypoxemia/hypoventilation**. If the lungs were compensating correctly, PO2 would typically be normal or elevated due to hyperpnea. The presence of respiratory failure alongside low bicarbonate confirms a **Mixed Respiratory and Metabolic Acidosis**. #### Why Incorrect Options are Wrong: * **Respiratory Acidosis (A):** While the low PO2 suggests a respiratory issue, the low HCO3– (13) cannot be a compensation for respiratory acidosis (which would cause HCO3– to rise). * **Metabolic Alkalosis (B) & Respiratory Alkalosis (C):** Both are ruled out immediately because the pH (7.2) is acidic, not alkaline (>7.45). #### High-Yield Clinical Pearls for NEET-PG: * **Mixed Disorders:** Suspect a mixed disorder when the compensation is inadequate or opposite to what is expected. * **Winter’s Formula:** Expected $PCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2$. If the actual $PCO_2$ is higher than expected, a concurrent respiratory acidosis exists. * **Common Scenario:** Mixed acidosis is frequently seen in **cardiopulmonary arrest** (lactic acidosis + respiratory failure) or severe **septic shock**.

Q: Which of the following is FALSE regarding bicarbonate (HCO3-)?

A 7.5% solution of bicarbonate contains 2 nanomoles.. ### Explanation **Why Option C is False (The Correct Answer):** The statement regarding the concentration of a 7.5% Sodium Bicarbonate ($NaHCO_3$) solution is mathematically incorrect. In medical pharmacology, a **7.5% solution** means 7.5 grams of the solute in 100 mL of solution. * **Calculation:** 7.5 g/100 mL is equivalent to 75 g/L. * The molecular weight of $NaHCO_3$ is approximately 84 g/mol. * Therefore, 75 g/L ÷ 84 g/mol ≈ **0.89 mol/L (or 890 millimoles/L)**. The claim that it contains "2 nanomoles" is an extreme underestimate, as the concentration is actually in the molar/millimolar range. **Analysis of Other Options:** * **Option A:** This is **True**. The normal range for arterial plasma bicarbonate is **22–28 mEq/L** (average ~24-25 mEq/L). It is the most important extracellular buffer. * **Option B:** This is **True**. Intracellular bicarbonate concentration is significantly lower than extracellular levels, typically averaging around **10 mEq/L** due to the negative resting membrane potential of cells which tends to repel anions. * **Option C:** This is **True**. In the proximal convoluted tubule (PCT), **Carbonic Anhydrase (Type IV)** on the brush border facilitates reabsorption, while **Carbonic Anhydrase (Type II)** in the cytoplasm helps generate "new" bicarbonate, which is then transported into the blood. **High-Yield Clinical Pearls for NEET-PG:** * **Bicarbonate Reabsorption:** 85% occurs in the PCT. It cannot be reabsorbed directly; it must first be converted to $CO_2$ and $H_2O$ by Carbonic Anhydrase. * **Henderson-Hasselbalch Equation:** $pH = 6.1 + \log ([HCO_3^-] / 0.03 \times PCO_2)$. * **Anion Gap:** Calculated as $Na^+ - (Cl^- + HCO_3^-)$. Normal is 8–12 mEq/L. Bicarbonate levels drop in High Anion Gap Metabolic Acidosis (HAGMA) as it is consumed buffering organic acids.

Q: A female patient presents to the casualty department following an injury. Her arterial blood gas (ABG) analysis shows a low pH, high pCO2, and normal bicarbonate. What is the diagnosis?

Respiratory acidosis. ### Explanation **1. Understanding the Correct Answer (Respiratory Acidosis)** The diagnosis of acid-base disorders follows a systematic approach. In this case: * **Low pH ( 45 mmHg):** Carbon dioxide is a volatile acid. An elevation in $pCO_2$ (hypercapnia) indicates that the primary cause of the acidosis is respiratory (hypoventilation). * **Normal Bicarbonate ($HCO_3^-$):** This suggests an **acute** process. In acute respiratory acidosis, the kidneys have not yet had time (usually takes 24–48 hours) to compensate by retaining bicarbonate. **2. Why Other Options are Incorrect** * **Respiratory Alkalosis:** This would present with a **high pH** and a **low $pCO_2$** (usually due to hyperventilation). * **Metabolic Acidosis:** While the pH would be low, the primary driver would be a **low $HCO_3^-$**. The $pCO_2$ would typically be low or normal (due to respiratory compensation/Kussmaul breathing). * **Metabolic Alkalosis:** This would present with a **high pH** and a **high $HCO_3^-$**. **3. NEET-PG High-Yield Pearls** * **The "Golden Rule":** If pH and $pCO_2$ move in **opposite** directions, the primary problem is **Respiratory**. If they move in the **same** direction, it is **Metabolic** (ROME: Respiratory Opposite, Metabolic Equal). * **Clinical Correlation:** In a trauma/injury setting, acute respiratory acidosis is often caused by chest wall injuries (flail chest), pain-induced splinting, or head injuries depressing the respiratory center. * **Compensation:** For every 10 mmHg rise in $pCO_2$, $HCO_3^-$ rises by 1 mEq/L in acute cases and 3.5–4 mEq/L in chronic cases (e.g., COPD).

Q: If blood gas analysis reveals pH = 7.52, pCO2 = 30, and pO2 = 105, this will be compensated by?

Compensatory metabolic acidosis. ### Explanation To solve acid-base problems, follow a systematic three-step approach: **1. Identify the Primary Disturbance** * **pH = 7.52:** Normal pH is 7.35–7.45. Since it is >7.45, the primary condition is **Alkalosis**. * **pCO₂ = 30 mmHg:** Normal pCO₂ is 35–45 mmHg. A low pCO₂ (hypocapnia) causes alkalosis. * **Conclusion:** The primary disturbance is **Respiratory Alkalosis**. **2. Determine the Compensation** The body always attempts to return the pH toward normal by moving the opposite system in the same direction as the primary change. * In **Respiratory Alkalosis** (low pCO₂), the kidneys compensate by **excreting HCO₃⁻** (bicarbonate) and retaining H⁺. * Decreasing the base (HCO₃⁻) creates a **Metabolic Acidosis** to counter the respiratory alkalosis. --- ### Why the other options are incorrect: * **Option A & B:** Compensation is never performed by the same system that caused the primary derangement. Since the primary issue is respiratory, the compensation must be metabolic (renal). * **Option D:** Adding a metabolic alkalosis to a respiratory alkalosis would worsen the pH deviation, potentially leading to fatal alkalinity. --- ### NEET-PG High-Yield Pearls: * **The "Same Direction" Rule:** In simple acid-base disorders, the pCO₂ and HCO₃⁻ always move in the same direction (both up or both down) if compensation is occurring. * **Speed of Compensation:** Respiratory compensation (for metabolic issues) starts within minutes. Renal compensation (for respiratory issues) is slow, taking **2–5 days** to reach maximal effect. * **Expected Compensation in Acute Respiratory Alkalosis:** For every 10 mmHg drop in pCO₂, HCO₃⁻ drops by **2 mEq/L**. In chronic cases, it drops by **4–5 mEq/L**.

Q: In an exhausted child with severe bronchiolitis, for every 10 mm Hg increase in PCO2, how many mEq of bicarbonate will increase?

1. **Explanation:** The question tests the understanding of acid-base compensation in **Acute Respiratory Acidosis**. Bronchiolitis in an "exhausted child" indicates impending respiratory failure, leading to the rapid accumulation of $CO_2$. **1. Why Option D (1) is Correct:** In **Acute Respiratory Acidosis**, the body relies on immediate chemical buffering (primarily by hemoglobin and intracellular proteins) because the kidneys take 24–72 hours to mount a significant metabolic response. * **Rule of Thumb:** For every **10 mm Hg rise in $PaCO_2$**, the Bicarbonate ($HCO_3^-$) increases by **1 mEq/L**. * In this scenario, the child is acutely exhausted, meaning the compensation is in the acute phase, making 1 mEq/L the correct physiological response. **2. Why Other Options are Incorrect:** * **Option B (4):** This represents the compensation for **Chronic Respiratory Acidosis** (e.g., COPD or chronic neuromuscular weakness). In chronic states, the kidneys have had time to retain more bicarbonate. For every 10 mm Hg rise in $PaCO_2$, $HCO_3^-$ increases by **3.5 to 4 mEq/L**. * **Options A (2) and C (8):** These do not correspond to standard physiological compensation ratios for respiratory acidosis. **3. NEET-PG High-Yield Pearls:** * **Acute Respiratory Acidosis:** $\Delta HCO_3 = 1$ for every $10 \uparrow PaCO_2$. * **Chronic Respiratory Acidosis:** $\Delta HCO_3 = 3.5 - 4$ for every $10 \uparrow PaCO_2$. * **Clinical Sign:** In bronchiolitis, a "normal" $PaCO_2$ (40 mm Hg) in a child with increased work of breathing is a **danger sign** of impending respiratory failure; it should normally be low due to hyperventilation. * **Management:** If a child is "exhausted" with rising $PCO_2$, they require immediate respiratory support (CPAP or Intubation).

Q: In metabolic alkalosis, which statement is true about urinary excretion?

Decreased excretion of ammonia. ### Explanation **Core Concept:** In metabolic alkalosis, the body’s primary goal is to conserve hydrogen ions ($H^+$) and excrete excess bicarbonate ($HCO_3^-$) to restore a normal pH. The kidneys achieve this by reducing the secretion of $H^+$ into the tubular lumen. Since the formation and excretion of **ammonia ($NH_3$)** and **ammonium ($NH_4^+$)** are directly dependent on the availability of $H^+$ ions in the tubular fluid (to "trap" ammonia as ammonium), a state of alkalosis leads to a significant **decrease in renal ammoniagenesis** and ammonia excretion. **Analysis of Options:** * **D (Correct):** As $H^+$ secretion decreases during alkalosis, the conversion of $NH_3$ to $NH_4^+$ in the distal tubule diminishes, leading to decreased urinary ammonia excretion. * **A (Incorrect):** Increased $NH_3$ excretion is a hallmark of **metabolic acidosis**, where the kidneys ramp up ammonia production to buffer and eliminate excess acid. * **B & C (Incorrect):** Aceto-acetic acid and beta-hydroxybutyric acid are **ketoacids**. Their presence in urine (ketonuria) is associated with metabolic **acidosis** (e.g., Diabetic Ketoacidosis or starvation), not alkalosis. In alkalosis, the body is not in a state of pathological ketoacid production. **Clinical Pearls for NEET-PG:** * **Ammoniagenesis:** Occurs primarily in the **proximal convoluted tubule (PCT)** via the deamination of **Glutamine**. * **Urine pH:** In metabolic alkalosis, the urine is typically **alkaline** (pH > 7.0) as the kidney dumps $HCO_3^-$. * **Paradoxical Aciduria:** A high-yield exception where a patient has metabolic alkalosis but excretes acidic urine. This occurs in **hypokalemic, hypochloremic metabolic alkalosis** (e.g., persistent vomiting) because the kidney prioritizes volume and sodium conservation over pH balance.

Question 1

Interpret the following lab values: pH 7.42, PaCO2 25 mmHg, HCO3- 18 mEq/L.

Accepted Answer

Respiratory alkalosis, fully compensated

Answer

Metabolic acidosis, partially compensated

Answer

Respiratory acidosis, partially compensated

Answer

Respiratory acidosis, fully compensated

Question 2

A patient has the following arterial blood gas values: pH = 7.30, pCO2 = 38 mmHg, HCO3 = 18 mEq/L. What is the most likely diagnosis?

Accepted Answer

Metabolic acidosis with respiratory acidosis

Answer

Metabolic acidosis with compensatory respiratory alkalosis

Answer

Respiratory acidosis with compensatory metabolic alkalosis

Answer

Respiratory alkalosis with compensatory metabolic acidosis

Question 3

All of the following are seen in persistent vomiting EXCEPT?

Accepted Answer

Decreased K+ in urine

Answer

Hypokalemia

Answer

Elevated pH of blood

Answer

Metabolic alkalosis

Question 4

A patient presents with a pH of 7.2, PO2 of 50 mm Hg, and HCO3– of 13 mEq/L. These values are most consistent with which of the following acid-base disturbances?

Accepted Answer

Mixed respiratory and metabolic acidosis

Answer

Respiratory acidosis

Answer

Metabolic alkalosis

Answer

Respiratory alkalosis

Question 5

Metabolic alkalosis is associated with which of the following?

Accepted Answer

Acetazolamide

Answer

Fanconi's anemia

Answer

Hypocalcemia

Answer

Triamterene

Question 6

Which of the following is FALSE regarding bicarbonate (HCO3-)?

Accepted Answer

A 7.5% solution of bicarbonate contains 2 nanomoles.

Answer

Extracellular concentration is approximately 25 mEq/L.

Answer

Intracellular concentration is approximately 10 mEq/L.

Answer

In the kidney, bicarbonate is reabsorbed and produced by carbonic anhydrase.

Question 7

A female patient presents to the casualty department following an injury. Her arterial blood gas (ABG) analysis shows a low pH, high pCO2, and normal bicarbonate. What is the diagnosis?

Accepted Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Question 8

If blood gas analysis reveals pH = 7.52, pCO2 = 30, and pO2 = 105, this will be compensated by?

Accepted Answer

Compensatory metabolic acidosis

Answer

Compensatory respiratory acidosis

Answer

Compensatory respiratory alkalosis

Answer

Compensatory metabolic alkalosis

Question 9

In an exhausted child with severe bronchiolitis, for every 10 mm Hg increase in PCO2, how many mEq of bicarbonate will increase?

Accepted Answer

1

Answer

2

Answer

4

Answer

8

Question 10

In metabolic alkalosis, which statement is true about urinary excretion?

Accepted Answer

Decreased excretion of ammonia

Answer

Increased excretion of NH3

Answer

Decreased excretion of aceto-acetic acid

Answer

Excretion of betahydroxy butyric acid

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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