Acid-Base Balance — MCQs

Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 71: What acid-base status is suggested by the given parameters?

A. Metabolic acidosis and metabolic alkalosis
B. Metabolic acidosis with partial respiratory compensation (Correct Answer)
C. Respiratory acidosis with partial metabolic compensation
D. Respiratory acidosis

Explanation: ***Metabolic acidosis with partial respiratory compensation*** - **Low pH** indicates acidemia, **low HCO3-** confirms the primary disorder is metabolic acidosis, and **low PaCO2** represents partial respiratory compensation through hyperventilation. - The **respiratory system compensates** by eliminating CO2 to help normalize pH, but compensation is rarely complete, hence "partial." *Metabolic acidosis and metabolic alkalosis* - This describes a **mixed acid-base disorder** with both primary metabolic conditions occurring simultaneously. - The parameters show a single primary disorder (metabolic acidosis) with compensation, not two opposing metabolic processes. *Respiratory acidosis with partial metabolic compensation* - **Respiratory acidosis** would show **elevated PaCO2** as the primary abnormality, not the low PaCO2 seen here. - **Metabolic compensation** would involve **elevated HCO3-** retention by kidneys, contradicting the low bicarbonate levels. *Respiratory acidosis* - **Pure respiratory acidosis** requires **elevated PaCO2** with normal or slightly elevated HCO3- (acute) or significantly elevated HCO3- (chronic). - The **low PaCO2** in these parameters rules out any form of respiratory acidosis as the primary disorder.

Question 72: A patient is experiencing respiratory acidosis due to brain trauma. Which of the following lab values correlates with this acid imbalance?

A. Potassium level of 6.0 mEq/L (Correct Answer)
B. Potassium level of 2.5 mEq/L
C. Potassium level of 5.0 mEq/L
D. Potassium level of 3.5 mEq/L

Explanation: **Explanation:** The core concept here is the relationship between **hydrogen ions ($H^+$) and potassium ions ($K^+$)** across the cell membrane. In states of acidosis (whether respiratory or metabolic), there is an excess of $H^+$ ions in the extracellular fluid (ECF). To buffer this acidity, the body shifts $H^+$ ions into the intracellular compartment. To maintain electroneutrality, $K^+$ ions move out of the cells and into the ECF. This shift results in **hyperkalemia**. **Why Option A is correct:** A potassium level of **6.0 mEq/L** represents hyperkalemia (normal range: 3.5–5.0 mEq/L). In respiratory acidosis caused by brain trauma (hypoventilation), the accumulation of $CO_2$ leads to increased $H^+$. The subsequent $H^+/K^+$ exchange causes serum potassium levels to rise. **Why the other options are incorrect:** * **Option B (2.5 mEq/L):** This indicates severe hypokalemia, which is typically associated with alkalosis (where $K^+$ shifts into cells) or renal/GI losses. * **Options C & D (5.0 and 3.5 mEq/L):** These are within the normal physiological range. While a patient *could* theoretically start with a low-normal potassium, the most characteristic laboratory correlation for an acute acidotic state in a standardized exam is an elevated value. **High-Yield Clinical Pearls for NEET-PG:** * **Rule of Thumb:** For every 0.1 unit decrease in arterial pH, the serum potassium concentration increases by approximately 0.6 mEq/L. * **Exception:** In metabolic acidosis caused by organic acids (e.g., lactic acidosis or ketoacidosis), the hyperkalemia is often less pronounced than in mineral acidosis (e.g., HCl infusion) because organic anions can follow $H^+$ into the cell, reducing the need for $K^+$ exchange. * **Brain Trauma Connection:** Trauma to the medulla or pons can disrupt the respiratory centers, leading to hypoventilation, $CO_2$ retention, and respiratory acidosis.

Question 73: In the following flow diagram, Box A represents:

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

Explanation: ***Metabolic alkalosis*** - Box A is positioned in the **alkalotic branch** (pH > 7.45) where the primary disturbance is **elevated plasma HCO3-** (bicarbonate). - This represents a **metabolic disorder** causing alkalosis through increased bicarbonate or loss of hydrogen ions. *Metabolic acidosis* - Would be positioned in the **acidotic branch** (pH < 7.35) with **decreased HCO3-** as the primary disturbance. - Characterized by **bicarbonate loss** or **acid accumulation**, opposite to Box A's location. *Respiratory alkalosis* - Located in the **alkalotic branch** but with **decreased pCO2** as the primary parameter, not elevated HCO3-. - Results from **hyperventilation** causing excessive CO2 elimination, not bicarbonate elevation. *Respiratory acidosis* - Positioned in the **acidotic branch** (pH < 7.35) with **elevated pCO2** as the primary disturbance. - Caused by **hypoventilation** or lung disease leading to CO2 retention, opposite to alkalotic conditions.

Question 74: A patient's arterial blood gas analysis reveals pH 7.2, HCO3 36mmol/L, and pCO2 60mm of Hg. What is the primary acid-base abnormality?

A. Respiratory acidosis with compensatory metabolic alkalosis
B. Respiratory acidosis (Correct Answer)
C. Respiratory alkalosis with compensatory metabolic acidosis
D. Respiratory acidosis with compensatory metabolic acidosis

Explanation: ### Explanation To identify the primary acid-base abnormality, follow a systematic three-step approach: 1. **Check the pH:** The normal range is 7.35–7.45. A pH of **7.2** indicates **Acidosis**. 2. **Check the $pCO_2$:** The normal range is 35–45 mmHg. Here, $pCO_2$ is **60 mmHg** (elevated). Since $CO_2$ is an acid, its elevation correlates with the acidic pH, confirming a **Respiratory** origin. 3. **Check the $HCO_3^-$:** The normal range is 22–26 mmol/L. Here, $HCO_3^-$ is **36 mmol/L** (elevated). This elevation is a compensatory mechanism by the kidneys to buffer the acidosis. **Why Option B is Correct:** The primary insult is the retention of $CO_2$ (Respiratory Acidosis). While compensation is present, the question asks for the *primary* abnormality. **Why Other Options are Incorrect:** * **Option A:** While metabolic compensation is occurring, "Respiratory acidosis" is the fundamental diagnosis. In many exams, if the pH is still abnormal, we identify the primary cause first. * **Option C:** Incorrect because the pH is acidic (<7.35), not alkalotic. * **Option D:** This is physiologically impossible; the body would not compensate for acidosis by creating more acidosis (lowering bicarbonate). ### NEET-PG High-Yield Pearls * **The "Match" Rule:** If the pH and $pCO_2$ move in opposite directions, the primary problem is Respiratory. If pH and $HCO_3^-$ move in the same direction, it is Metabolic. * **Compensation Limits:** In acute respiratory acidosis, $HCO_3^-$ rises by 1 mEq/L for every 10 mmHg rise in $pCO_2$. In chronic cases (like COPD), it rises by 3.5–4 mEq/L. * **Golden Rule:** The body never over-compensates; the pH will never return fully to normal or "cross over" to the other side.

Question 75: Which of the following hormones does NOT modify growth?

A. Growth hormone / corticotrophin
B. Vasopressin (Correct Answer)
C. Insulin
D. Prolactin

Explanation: **Explanation:** Growth is a complex physiological process regulated by a specific set of hormones that influence protein synthesis, cell division, and skeletal maturation. **Why Vasopressin is the Correct Answer:** **Vasopressin (Antidiuretic Hormone/ADH)** is primarily involved in water homeostasis and vascular tone. It acts on the V2 receptors in the renal collecting ducts to increase water reabsorption and V1 receptors to cause vasoconstriction. It has **no physiological role** in stimulating or modifying systemic growth or skeletal development. **Analysis of Other Options:** * **Growth Hormone (GH) / Corticotrophin:** GH is the primary stimulator of postnatal growth via IGF-1. Corticotrophin (ACTH) influences growth indirectly; while essential in physiological amounts, chronic excess (Cushing’s) actually inhibits growth by antagonizing GH and causing protein catabolism. * **Insulin:** Insulin is a potent growth promoter, especially during **fetal development**. It stimulates amino acid uptake and protein synthesis. Children with insulin deficiency (Type 1 Diabetes) often exhibit stunted growth (Mauriac syndrome). * **Prolactin:** Prolactin belongs to the same structural family as Growth Hormone. While its primary role is lactation, it has weak GH-like effects and can influence the proliferation of specific tissues, including immune cells and mammary tissue. **High-Yield Clinical Pearls for NEET-PG:** * **Fetal Growth:** Primarily regulated by **Insulin** and **IGF-2**. Growth Hormone is *not* essential for intrauterine growth. * **Postnatal Growth:** Primarily regulated by **GH, IGF-1, and Thyroid hormones**. * **Pubertal Growth Spurt:** Driven by the synergistic action of **Androgens/Estrogens** and GH. * **Thyroid Hormone:** Essential for bone maturation and CNS development; deficiency leads to cretinism (stunted growth and mental retardation).

Question 76: Antral obstruction with vomiting is NOT characterized by which of the following?

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

Explanation: **Explanation:** Antral obstruction (such as Gastric Outlet Obstruction) leads to persistent vomiting of gastric contents. Gastric juice is rich in hydrochloric acid (HCl), potassium, and sodium. Therefore, the hallmark of this condition is **Metabolic Alkalosis**, not acidosis. **1. Why Acidosis is the Correct Answer (The Exception):** Vomiting results in the massive loss of hydrogen ions ($H^+$) from the stomach. To compensate, the body shifts bicarbonate ($HCO_3^-$) into the extracellular fluid. This leads to **Metabolic Alkalosis** (specifically, Hypochloremic Hypokalemic Metabolic Alkalosis). Therefore, "Acidosis" is the incorrect clinical finding. **2. Analysis of Other Options:** * **Hypochloremia (B):** Gastric juice contains high concentrations of $Cl^-$. Loss of HCl directly leads to low serum chloride. * **Hypokalemia (A):** Potassium is lost directly in vomitus. Furthermore, in alkalosis, $K^+$ shifts intracellularly, and the kidneys excrete $K^+$ in exchange for $H^+$ to conserve acid, worsening the deficit. * **Hyponatremia (D):** Sodium is lost in the gastric juice. Additionally, volume depletion triggers ADH release, which causes water retention and further dilutes serum sodium. **Clinical Pearls for NEET-PG:** * **Paradoxical Aciduria:** In severe cases, despite systemic alkalosis, the urine becomes acidic. This occurs because the kidney prioritizes volume expansion (via Aldosterone) by reabsorbing $Na^+$. To maintain electrical neutrality without $Cl^-$, the kidney is forced to excrete $H^+$. * **Classic Presentation:** A patient with a history of peptic ulcer disease presenting with projectile, non-bilious vomiting. * **Treatment:** The fluid of choice is **0.9% Normal Saline** (to replace $Cl^-$ and volume) with **Potassium supplementation**.

Question 77: Arterial blood gas analysis of a patient admitted to the medical emergency is as follows: pH = 7.2, HCO3 = 38 mmol/L, pCO2 = 56 mm Hg. This indicates:

A. Metabolic acidosis
B. Metabolic acidosis with respiratory compensation
C. Respiratory acidosis
D. Respiratory acidosis with renal compensation (Correct Answer)

Explanation: ### Explanation To solve any Acid-Base question, follow a systematic three-step approach: **1. Determine the Primary Disorder (pH):** The normal pH range is 7.35–7.45. A pH of **7.2** indicates **Acidosis**. **2. Identify the Cause (pCO2 vs. HCO3):** * **pCO2:** Normal is 40 mmHg. Here, it is **56 mmHg** (High). High CO2 causes respiratory acidosis. * **HCO3:** Normal is 24 mmol/L. Here, it is **38 mmol/L** (High). High bicarbonate is a basic/alkaline change. Since the pH is acidic, the elevated pCO2 must be the primary driver. Therefore, the primary diagnosis is **Respiratory Acidosis**. **3. Evaluate Compensation:** In primary respiratory acidosis, the kidneys compensate by retaining HCO3 to buffer the acid. Since the HCO3 is significantly elevated (38 mmol/L), **renal compensation** is occurring. It is "partial" compensation because the pH has not yet returned to the normal range. --- ### Why Other Options are Wrong: * **A & B (Metabolic Acidosis):** In metabolic acidosis, the primary change is a *low* HCO3 (<22 mmol/L) and a *low* pH. Here, HCO3 is high. * **C (Respiratory Acidosis):** While technically true, it is incomplete. "Respiratory acidosis" alone implies an acute state without compensation. The significantly high HCO3 (38 mmol/L) confirms that renal compensation is active. --- ### High-Yield Clinical Pearls for NEET-PG: * **Compensation Rule:** The body never over-compensates. If the pH is <7.40, the primary process is acidosis; if >7.40, it is alkalosis. * **Time Frame:** Renal compensation for respiratory disorders is slow, taking **3–5 days** to reach maximal effect. * **Expected HCO3 in Chronic Respiratory Acidosis:** For every 10 mmHg rise in pCO2, HCO3 should rise by **3.5–4 mmol/L**. (In this case: 16 mmHg rise in CO2 $\approx$ 6.4 mmol/L rise in HCO3. The actual rise is 14 mmol/L, suggesting a possible concurrent metabolic alkalosis or long-standing chronic state).

Question 78: Kussmaul's breathing is characteristic of which condition?

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

Explanation: **Explanation:** **Kussmaul’s breathing** is a deep, rapid, and labored breathing pattern that serves as a compensatory mechanism for **Metabolic Acidosis** (Option A). The underlying physiological concept is the **Chemoreceptor Reflex**. In metabolic acidosis, there is an accumulation of non-volatile acids (e.g., ketones in DKA or lactic acid) and a drop in arterial pH. This acidity stimulates peripheral chemoreceptors (carotid and aortic bodies) and the central chemosensitive area. To compensate, the respiratory center increases the rate and depth of ventilation to "blow off" excess Carbon Dioxide ($CO_2$). Since $CO_2$ acts as a volatile acid, reducing its levels helps raise the blood pH back toward the normal range (7.35–7.45). **Why other options are incorrect:** * **Metabolic Alkalosis (B):** The body compensates by **hypoventilation** (slow, shallow breathing) to retain $CO_2$ and lower the pH. * **Respiratory Acidosis (C):** This is caused by primary hypoventilation (e.g., COPD, opioid overdose). The lungs are the *source* of the problem, not the solution; therefore, Kussmaul’s breathing is absent. * **Respiratory Alkalosis (D):** This results from hyperventilation (e.g., anxiety, high altitude). While the breathing is fast, it is the *cause* of the alkalosis, not a compensatory response to a metabolic derangement. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Association:** Kussmaul’s breathing is most famously associated with **Diabetic Ketoacidosis (DKA)**. * **Mnemonic for Causes:** **KUSSMAUL** (Ketones, Uremia, Sepsis, Salicylates, Methanol, Aldehydes, Uremia, Lactic acidosis). * **Distinction:** Unlike Cheyne-Stokes breathing (which has periods of apnea), Kussmaul’s breathing is consistent, rhythmic, and gasping.

Question 79: What are the ECG changes seen in hypocalcemia?

A. ST segment depression
B. Prolongation of ST segment (Correct Answer)
C. Inversion of T wave
D. Prolongation of PR segment

Explanation: **Explanation:** The hallmark ECG finding in **hypocalcemia** is the **prolongation of the ST segment**, which leads to a prolonged **QT interval**. **1. Why the correct answer is right:** Calcium ions play a critical role in the plateau phase (Phase 2) of the cardiac action potential. In hypocalcemia, the reduced extracellular calcium concentration slows the influx of calcium through L-type calcium channels. This delays the completion of the plateau phase, thereby lengthening the duration of the ST segment. Since the T wave (repolarization) remains relatively unchanged, the overall effect is a **prolonged QT interval** (specifically due to the long ST segment). **2. Why the incorrect options are wrong:** * **ST segment depression:** This is typically associated with myocardial ischemia, hypokalemia, or digoxin effect, rather than calcium imbalances. * **Inversion of T wave:** This is a sign of myocardial ischemia, ventricular hypertrophy, or bundle branch blocks. In hypocalcemia, the T wave is usually upright and normal. * **Prolongation of PR segment:** This indicates a delay in AV node conduction, commonly seen in first-degree heart block or hyperkalemia, but not typically associated with hypocalcemia. **High-Yield Clinical Pearls for NEET-PG:** * **Hypocalcemia:** Prolonged ST segment $\rightarrow$ Prolonged QT interval (can predispose to Torsades de Pointes, though less common than in hypomagnesemia). * **Hypercalcemia:** Shortened ST segment $\rightarrow$ **Shortened QT interval** (the "Osborn wave" or J wave is more specific to hypothermia but can rarely be seen here). * **Mnemonic:** "Hypo-Long, Hyper-Short" (referring to the QT interval). * **Clinical Signs:** Look for Trousseau’s sign and Chvostek’s sign in the clinical stem.

Question 80: What is the commonest cause of metabolic alkalosis?

A. Cancer of the stomach
B. Pyloric stenosis (Correct Answer)
C. Small-bowel obstruction
D. Diuretics

Explanation: **Explanation:** **Pyloric stenosis** is the classic cause of metabolic alkalosis due to the persistent vomiting of gastric contents. Gastric juice is rich in hydrochloric acid (HCl). When a patient vomits (as seen in the projectile vomiting of congenital hypertrophic pyloric stenosis), there is a massive loss of hydrogen ions ($H^+$) and chloride ions ($Cl^-$). The underlying mechanism involves: 1. **Loss of $H^+$:** Leads to a direct increase in plasma bicarbonate ($HCO_3^-$). 2. **Hypochloremia:** Loss of $Cl^-$ forces the kidneys to reabsorb $HCO_3^-$ to maintain anionic balance. 3. **Contraction Alkalosis:** Dehydration triggers the Renin-Angiotensin-Aldosterone System (RAAS). Aldosterone promotes $Na^+$ reabsorption at the expense of $K^+$ and $H^+$ excretion in the distal tubule, further worsening the alkalosis (Paradoxical Aciduria). **Analysis of Incorrect Options:** * **A. Cancer of the stomach:** While it can cause obstruction, it is not the *most common* or classic association compared to the specific pathology of pyloric stenosis. * **C. Small-bowel obstruction:** Obstruction distal to the ampulla of Vater results in the loss of both acidic gastric juice and alkaline pancreatic/biliary secretions, often leading to a **neutral pH or metabolic acidosis** (if primarily lower bowel). * **D. Diuretics:** Loop and thiazide diuretics do cause metabolic alkalosis, but in the context of clinical exams and classic surgical/pediatric presentations, pyloric stenosis remains the "textbook" commonest cause cited for profound metabolic alkalosis. **High-Yield NEET-PG Pearls:** * The characteristic electrolyte profile in pyloric stenosis is **Hypochloremic, Hypokalemic, Metabolic Alkalosis with Paradoxical Aciduria.** * **Paradoxical Aciduria** occurs because the body prioritizes volume expansion (reabsorbing $Na^+$) over pH balance, leading to $H^+$ secretion in the urine despite systemic alkalosis.

Practice by Chapter

Acid-Base Chemistry

Practice Questions

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

Practice Questions

Respiratory Acidosis and Alkalosis

Practice Questions

Metabolic Acidosis and Alkalosis

Practice Questions

Mixed Acid-Base Disorders

Practice Questions

Compensatory Mechanisms

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Clinical Assessment of Acid-Base Status

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