Acid-Base Balance Practice Questions

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

Metabolic acidosis. **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.

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

Metabolic alkalosis. **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).

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

3-4 days. **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 ( 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).

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

Respiratory acidosis, not fully compensated. **Explanation:** To solve acid-base problems, follow a systematic three-step approach: 1. **pH Status:** The pH is **decreased ( 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.

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

Metabolic acidosis. **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: $pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$.

Q: Severe anxiety causes which of the following acid-base disturbances?

Respiratory alkalosis. **Explanation:** **Why Respiratory Alkalosis is the correct answer:** Severe anxiety or panic attacks trigger the sympathetic nervous system, leading to **hyperventilation** (increased rate and depth of breathing). This excessive ventilation results in the "washing out" of Carbon Dioxide ($CO_2$) from the lungs. Since $CO_2$ acts as a volatile acid in the blood (forming carbonic acid), its rapid depletion leads to a decrease in the partial pressure of arterial $CO_2$ ($PaCO_2 < 35\ mmHg$) and a subsequent **increase in blood pH**, resulting in respiratory alkalosis. **Analysis of Incorrect Options:** * **Respiratory Acidosis:** This occurs due to **hypoventilation** (e.g., COPD, opioid overdose), where $CO_2$ is retained, lowering the pH. Anxiety causes the exact opposite (hyperventilation). * **Metabolic Acidosis:** This is caused by a primary decrease in bicarbonate ($HCO_3^-$) or an accumulation of fixed acids (e.g., Diabetic Ketoacidosis, Renal failure). Anxiety does not acutely affect metabolic acid production. * **Metabolic Alkalosis:** This results from a primary gain of $HCO_3^-$ or loss of $H^+$ ions (e.g., persistent vomiting, diuretic use), which is unrelated to the respiratory changes seen in anxiety. **Clinical Pearls for NEET-PG:** * **The "Paper Bag" Treatment:** Rebreathing into a paper bag during an anxiety attack helps the patient inhale expired $CO_2$, restoring $PaCO_2$ levels and correcting the pH. * **Chvostek’s and Trousseau’s Signs:** Acute respiratory alkalosis causes a shift of ionized calcium to bind with albumin. This sudden **hypocalcemia** leads to carpopedal spasms and tetany, often seen in hyperventilating patients. * **Rule of Thumb:** In acute respiratory alkalosis, for every 10 mmHg decrease in $PaCO_2$, the $pH$ increases by approximately 0.08 units.

Q: Hyperaldosteronism is characterized by which acid-base disturbance?

Metabolic alkalosis. In hyperaldosteronism (e.g., Conn’s Syndrome), the excess mineralocorticoid acts on the **Principal cells** and **Alpha-intercalated cells** of the distal convoluted tubule and collecting duct [1]. **Why Metabolic Alkalosis is Correct:** Aldosterone increases the reabsorption of Sodium ($Na^+$) in exchange for Potassium ($K^+$) and Hydrogen ions ($H^+$) [2]. This occurs via two primary mechanisms: 1. **Direct $H^+$ secretion:** Aldosterone stimulates the $H^+$-ATPase pump in the alpha-intercalated cells, directly secreting protons into the urine [2]. 2. **Indirect effect:** The reabsorption of $Na^+$ creates a negative luminal potential, which "pulls" $K^+$ and $H^+$ out of the cells into the tubular lumen. The loss of $H^+$ ions leads to an increase in serum bicarbonate levels, resulting in **Metabolic Alkalosis** [1]. This is typically associated with **Hypokalemia** (due to $K^+$ wasting) [2]. **Why Incorrect Options are Wrong:** * **Metabolic Acidosis:** This occurs in conditions of aldosterone deficiency (e.g., Addison’s disease or Type 4 RTA), where $H^+$ cannot be excreted. * **Respiratory Acidosis/Alkalosis:** These are primary disturbances of $CO_2$ regulation by the lungs. Hyperaldosteronism is a primary metabolic/renal pathology. **High-Yield Clinical Pearls for NEET-PG:** * **The Triad:** Hypertension + Hypokalemia + Metabolic Alkalosis = Primary Hyperaldosteronism [1]. * **Aldosterone Escape:** Despite high sodium reabsorption, patients with primary hyperaldosteronism do not usually have clinical edema due to "ANP-mediated escape" [3]. * **Screening:** The best initial test is the **Plasma Aldosterone to Plasma Renin Activity (ARR) ratio**. A ratio >20-30 is suggestive.

Q: Which of the following is referred to as the alkali reserve of the body?

Bicarbonate. **Explanation:** The term **"Alkali Reserve"** refers to the concentration of **Bicarbonate ions ($HCO_3^-$)** in the blood. It represents the body's primary buffering capacity against non-volatile (fixed) acids like lactic acid or ketone bodies. **1. Why Bicarbonate is the Correct Answer:** In the Bicarbonate-Carbonic Acid buffer system (the most important extracellular buffer), the ratio of $HCO_3^-$ to $H_2CO_3$ is maintained at **20:1** at a physiological pH of 7.4. Because bicarbonate is present in such high concentrations and can be regulated by the kidneys, it acts as a "reserve" of base that can neutralize incoming hydrogen ions ($H^+$), preventing a drop in blood pH. **2. Why the Other Options are Incorrect:** * **Carbonic Acid ($H_2CO_3$):** This is the acidic component of the buffer system. It is formed when $CO_2$ dissolves in water. It represents the "acid" side of the balance, not the alkali reserve. * **Water ($H_2O$):** While water is the solvent for all biochemical reactions and participates in the formation of carbonic acid, it is neutral and does not function as a buffer or an alkali reserve. **High-Yield Clinical Pearls for NEET-PG:** * **Henderson-Hasselbalch Equation:** $pH = pKa + \log ([HCO_3^-] / [0.03 \times PCO_2])$. * **Normal Bicarbonate Levels:** 22–28 mEq/L. * **Metabolic Acidosis:** Characterized by a primary **decrease** in the alkali reserve ($HCO_3^-$). * **Anion Gap:** Useful in differentiating causes of metabolic acidosis; it is calculated as $Na^+ - (Cl^- + HCO_3^-)$. Normal range is 8–12 mEq/L.

Q: A 72-year-old male with diabetes mellitus presents with lethargy, disorientation, and Kussmaul respirations. Initial laboratory results show a high glucose level of 380 mg/dL and a venous blood pH of 7.3. Given normal bicarbonate levels of 22 to 28 mM and PCO2 values of 33 to 45 mmHg, which of the following additional test results would be consistent with the patient's pH and breathing pattern?

Bicarbonate of 15 mM and PCO2 of 30 mmHg. **Explanation:** The patient presents with classic signs of **Diabetic Ketoacidosis (DKA)**: hyperglycemia, lethargy, and **Kussmaul respirations** (deep, rapid breathing). In DKA, the accumulation of ketoacids leads to a **Metabolic Acidosis**, characterized by a low pH (<7.35) and low bicarbonate ($HCO_3^-$). 1. **Why Option B is correct:** The patient’s pH of 7.3 indicates acidosis. Kussmaul breathing is a physiological **respiratory compensation** where the lungs "blow off" $CO_2$ to raise the pH. Therefore, we expect a low $HCO_3^-$ (the primary metabolic cause) and a low $PCO_2$ (the respiratory compensation). Option B ($HCO_3^-$ 15 mM, $PCO_2$ 30 mmHg) reflects this compensated state. 2. **Why other options are incorrect:** * **Option A:** While it shows compensation, a $PCO_2$ of 10 mmHg is extreme and usually seen in much more severe acidosis (pH < 7.1). * **Option C:** Shows low $HCO_3^-$ but a normal $PCO_2$ (40 mmHg). This indicates a lack of respiratory compensation, which contradicts the clinical finding of Kussmaul respirations. * **Option D:** Shows a high $PCO_2$ (45 mmHg), which would indicate respiratory acidosis, further lowering the pH rather than compensating for it. **High-Yield NEET-PG Pearls:** * **Winters' Formula:** To check if respiratory compensation is adequate in metabolic acidosis: $Expected\ PCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$. * **Kussmaul Breathing:** A hallmark of metabolic acidosis (MUDPILES); it represents the body's attempt to induce a compensatory respiratory alkalosis. * **Anion Gap:** DKA always presents with a **High Anion Gap Metabolic Acidosis (HAGMA)** due to the presence of unmeasured anions (acetoacetate and beta-hydroxybutyrate).

Q: Given the lab values: pH 7.56, paCO2 20, HCO3 20. Interpret the acid-base status?

Respiratory alkalosis, partially compensated. ### Explanation To interpret any acid-base disorder, follow a systematic three-step approach: **1. Determine the Primary Disturbance (pH):** The normal pH range is 7.35–7.45. A pH of **7.56** indicates **Alkalemia**. **2. Identify the Cause (Respiratory vs. Metabolic):** * The **paCO2 is 20 mmHg** (Normal: 35–45). Low CO2 (hypocapnia) causes alkalosis. * The **HCO3 is 20 mEq/L** (Normal: 22–26). Low HCO3 causes acidosis. Since the low paCO2 matches the alkalemic pH, the primary diagnosis is **Respiratory Alkalosis**. **3. Determine Compensation:** In respiratory alkalosis, the kidneys compensate by excreting HCO3 to bring the pH back toward normal. * **Uncompensated:** HCO3 would be normal (24). * **Partially Compensated:** HCO3 has started to drop (as seen here, 20), but the **pH is still abnormal**. * **Fully Compensated:** HCO3 has dropped enough to return the pH to the 7.35–7.45 range. **Why Incorrect Options are Wrong:** * **Option A:** Incorrect because the HCO3 (20) is below the normal range, indicating the kidneys have already begun compensating. * **Option B:** Incorrect because the pH (7.56) is still outside the normal range. * **Option D:** Incorrect because a metabolic alkalosis would require a high pH and a high HCO3. ### High-Yield NEET-PG Pearls * **The "Rule of Thumb":** If the pH and the primary buffer (CO2 or HCO3) move in **opposite** directions, it is a respiratory disorder. If they move in the **same** direction, it is metabolic. * **Compensation Limits:** The body never "over-compensates." If the pH crosses the 7.40 midline, suspect a mixed acid-base disorder. * **Acute vs. Chronic:** In acute respiratory alkalosis, HCO3 drops by 2 mEq/L for every 10 mmHg drop in paCO2. In chronic cases, it drops by 4–5 mEq/L.

Question 1

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

Accepted Answer

Metabolic acidosis

Answer

Hyponatremia

Answer

Hypochloremia

Answer

Hypokalemia

Question 2

Hypokalemia is frequently associated with which acid-base disorder?

Accepted Answer

Metabolic alkalosis

Answer

Metabolic acidosis

Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Question 3

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

Accepted Answer

3-4 days

Answer

Less than 1 day

Answer

1-2 days

Answer

More than 7 days

Question 4

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

Accepted Answer

Respiratory acidosis, not fully compensated

Answer

Respiratory alkalosis, compensated

Answer

Respiratory acidosis, compensated

Answer

Metabolic alkalosis, uncompensated

Question 5

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

Accepted Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Question 6

Severe anxiety causes which of the following acid-base disturbances?

Accepted Answer

Respiratory alkalosis

Answer

Respiratory acidosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Question 7

Hyperaldosteronism is characterized by which acid-base disturbance?

Accepted Answer

Metabolic alkalosis

Answer

Metabolic acidosis

Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Question 8

Which of the following is referred to as the alkali reserve of the body?

Accepted Answer

Bicarbonate

Answer

Carbonic acid

Answer

Water

Answer

All of the above

Question 9

A 72-year-old male with diabetes mellitus presents with lethargy, disorientation, and Kussmaul respirations. Initial laboratory results show a high glucose level of 380 mg/dL and a venous blood pH of 7.3. Given normal bicarbonate levels of 22 to 28 mM and PCO2 values of 33 to 45 mmHg, which of the following additional test results would be consistent with the patient's pH and breathing pattern?

Accepted Answer

Bicarbonate of 15 mM and PCO2 of 30 mmHg

Answer

Bicarbonate of 5 mM and PCO2 of 10 mmHg

Answer

Bicarbonate of 15 mM and PCO2 of 40 mmHg

Answer

Bicarbonate of 20 mM and PCO2 of 45 mmHg

Question 10

Given the lab values: pH 7.56, paCO2 20, HCO3 20. Interpret the acid-base status?

Accepted Answer

Respiratory alkalosis, partially compensated

Answer

Respiratory alkalosis, uncompensated

Answer

Respiratory alkalosis, compensated

Answer

Metabolic alkalosis, partially compensated

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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