Acid-Base Balance Practice Questions

Q: Given a patient with pH = 7.27, HCO3 = 10 mEq/dl, and pCO2 = 23 mm Hg, what is the acid-base disorder?

Metabolic acidosis. To solve acid-base problems for NEET-PG, follow a systematic 3-step approach: **1. Analyze the pH:** The normal pH range is 7.35–7.45. A pH of **7.27** is less than 7.35, indicating **Acidosis**. This immediately eliminates options B and D. **2. Identify the Primary Cause:** * **Metabolic:** Look at Bicarbonate ($HCO_3^-$). Normal is 22–26 mEq/L. Here, $HCO_3^-$ is **10 mEq/L** (Low). Low bicarbonate causes acidosis. * **Respiratory:** Look at $pCO_2$. Normal is 35–45 mmHg. Here, $pCO_2$ is **23 mmHg** (Low). Low $pCO_2$ actually causes alkalosis. Since the low $HCO_3^-$ matches the acidic pH, the primary disorder is **Metabolic Acidosis**. **3. Evaluate Compensation:** The low $pCO_2$ (23 mmHg) represents the body’s respiratory attempt to compensate by "blowing off" acid to raise the pH back toward normal. **Why Incorrect Options are Wrong:** * **Metabolic Alkalosis:** Would present with a high pH (>7.45) and high $HCO_3^-$. * **Respiratory Acidosis:** Would present with a low pH ( 45 mmHg). * **Respiratory Alkalosis:** Would present with a high pH (>7.45) and low $pCO_2$. **High-Yield Clinical Pearls for NEET-PG:** * **Winters' Formula:** In metabolic acidosis, expected $pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2$. Here: $(1.5 \times 10) + 8 = 23$. Since the measured $pCO_2$ matches, it is a **pure metabolic acidosis with appropriate compensation.** * **Anion Gap:** Always calculate the Anion Gap ($Na^+ - [Cl^- + HCO_3^-]$) in metabolic acidosis to narrow the differential (e.g., MUDPILES for High Anion Gap). * **ROME Mnemonic:** **R**espiratory **O**pposite (pH and $CO_2$ move in opposite directions), **M**etabolic **E**qual (pH and $HCO_3^-$ move in the same direction).

Q: Which set of arterial blood gas values describes a heavy smoker with COPD who is becoming increasingly somnolent?

pH: 7.2; HCO3-: 30 mEq/L; PCO2: 60 mmHg. ### Explanation **1. Analysis of the Correct Answer (Option D)** The clinical scenario describes a patient with **COPD** (Chronic Obstructive Pulmonary Disease) who is becoming **somnolent**. This indicates **CO2 narcosis** due to severe hypoventilation. * **Respiratory Acidosis:** In COPD, impaired gas exchange leads to the retention of CO2 (Hypercapnia). A $PCO_2$ of 60 mmHg (>40 mmHg) confirms respiratory acidosis. * **Acidemia:** The pH of 7.2 (<7.35) confirms an uncompensated or partially compensated acidotic state. * **Renal Compensation:** In chronic respiratory acidosis, the kidneys retain $HCO_3^-$ to buffer the acidity. A $HCO_3^-$ of 30 mEq/L (Normal: 24 mEq/L) indicates partial metabolic compensation. **2. Analysis of Incorrect Options** * **Option A (pH 7.65, $HCO_3^-$ 48, $PCO_2$ 45):** This represents **Metabolic Alkalosis** (high pH and high $HCO_3^-$). This is not seen in COPD-induced respiratory failure. * **Option B (pH 7.5, $HCO_3^-$ 12, $PCO_2$ 20):** This represents **Respiratory Alkalosis** (high pH, low $PCO_2$) with renal compensation. This occurs in hyperventilation (e.g., anxiety or high altitude), not COPD. * **Option C (pH 7.4, $HCO_3^-$ 18, $PCO_2$ 40):** These values are near normal or represent a fully compensated mixed disorder, which does not fit the clinical picture of an acutely somnolent COPD patient. **3. High-Yield Clinical Pearls for NEET-PG** * **CO2 Narcosis:** High levels of $PaCO_2$ (typically >60-70 mmHg) act as a central nervous system depressant, leading to somnolence and coma. * **Compensation Rule:** In **Chronic** Respiratory Acidosis, for every 10 mmHg rise in $PCO_2$, $HCO_3^-$ increases by **3.5 to 4 mEq/L**. * **Winter’s Formula:** Used to calculate expected $PCO_2$ in metabolic acidosis: $PCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2$.

Q: Metabolic acidosis is associated with all of the following conditions EXCEPT?

Prolonged vomiting. **Explanation:** The correct answer is **B. Prolonged vomiting**. **1. Why Prolonged Vomiting is the Correct Answer:** Prolonged vomiting leads to **Metabolic Alkalosis**, not acidosis. Gastric juice is highly acidic due to high concentrations of hydrochloric acid (HCl). When a person vomits persistently, there is a significant loss of hydrogen ions ($H^+$) and chloride ions ($Cl^-$). To compensate for the loss of $Cl^-$, the kidneys retain bicarbonate ($HCO_3^-$), leading to an increase in blood pH. This condition is specifically termed **Hypochloremic Hypokalemic Metabolic Alkalosis**. **2. Analysis of Incorrect Options (Causes of Metabolic Acidosis):** * **Chronic Renal Failure (CRF):** The kidneys fail to excrete fixed acids (like phosphates and sulfates) and have a decreased ability to regenerate bicarbonate, leading to High Anion Gap Metabolic Acidosis (HAGMA). * **Starvation:** During starvation, the body shifts to fat metabolism, leading to the production of ketone bodies (acetoacetate and $\beta$-hydroxybutyrate). These are organic acids that lower blood pH. * **Uncontrolled Diabetes Mellitus:** Similar to starvation, a lack of insulin leads to **Diabetic Ketoacidosis (DKA)**. The accumulation of ketoacids results in a classic HAGMA. **3. High-Yield Clinical Pearls for NEET-PG:** * **Anion Gap:** Remember that DKA, Starvation, and Renal Failure are all causes of **High Anion Gap Metabolic Acidosis (HAGMA)**. * **Vomiting vs. Diarrhea:** Vomiting causes Alkalosis (loss of acid), whereas Diarrhea causes Metabolic Acidosis (loss of alkali/bicarbonate from lower GI secretions). * **Paradoxical Aciduria:** In severe vomiting, despite having systemic alkalosis, the kidneys may excrete acidic urine to conserve sodium and water—a high-yield concept often tested in exams.

Q: Which one of the following is most likely to result in contraction alkalosis?

Loop diuretics. **Explanation:** **Contraction Alkalosis** occurs when there is a significant loss of extracellular fluid (ECF) that is low in bicarbonate, leading to a relative increase in the concentration of the remaining bicarbonate. **1. Why Loop Diuretics are correct:** Loop diuretics (e.g., Furosemide) inhibit the Na⁺-K⁺-2Cl⁻ symporter in the thick ascending limb of the Loop of Henle. This leads to: * **Volume Depletion:** Massive loss of NaCl and water causes ECF contraction, concentrating the existing plasma bicarbonate. * **RAAS Activation:** Decreased ECF volume triggers the Renin-Angiotensin-Aldosterone System. Aldosterone increases H⁺ secretion in the distal tubule (via α-intercalated cells), further generating new bicarbonate and maintaining the alkalosis. **2. Analysis of Incorrect Options:** * **Laxatives:** Chronic laxative abuse typically results in **Normal Anion Gap Metabolic Acidosis** due to the loss of bicarbonate-rich intestinal secretions in the stool. * **Infant Formula:** Historically associated with "Chloride-deficient formula alkalosis," but it is not a primary cause of *contraction* alkalosis in a general physiological context compared to diuretics. * **Acetazolamide:** This is a Carbonic Anhydrase inhibitor. It causes increased excretion of bicarbonate in the urine, leading to **Metabolic Acidosis**, not alkalosis. **High-Yield Clinical Pearls for NEET-PG:** * **Common Causes:** The most common causes of contraction alkalosis are **Loop/Thiazide diuretics** and **Gastric suctioning/Vomiting** (loss of HCl). * **Chloride Status:** Contraction alkalosis is typically **Saline-responsive** (Urinary Cl⁻ < 20 mEq/L). Administering NaCl restores ECF volume and allows the kidneys to excrete the excess bicarbonate. * **Hypokalemia:** Often co-exists with contraction alkalosis due to secondary hyperaldosteronism.

Q: A normal-anion-gap metabolic acidosis occurs in patients with:

Diarrhoea. **Explanation:** Metabolic acidosis is categorized based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. A **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as hyperchloremic acidosis, occurs when the loss of bicarbonate ($HCO_3^-$) is compensated by a proportional increase in chloride ($Cl^-$) to maintain electroneutrality. **Why Diarrhoea is correct:** Lower gastrointestinal secretions are rich in bicarbonate. In **diarrhoea**, there is a direct physical loss of $HCO_3^-$ from the body. To balance the loss of these negative ions, the kidneys retain chloride, leading to hyperchloremia and a normal anion gap. **Analysis of Incorrect Options:** * **Diabetic Ketoacidosis (DKA):** Characterized by the accumulation of unmeasured anions (acetoacetate and beta-hydroxybutyrate), leading to a **High Anion Gap Metabolic Acidosis (HAGMA)**. * **Methyl Alcohol Poisoning:** Metabolism of methanol produces formic acid. These exogenous acid anions increase the anion gap (**HAGMA**). * **Acute Renal Failure:** Failure to excrete fixed acids (phosphates, sulfates) results in an accumulation of unmeasured anions, causing **HAGMA**. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for NAGMA (USED CARP):** **U**reterosigmoidostomy, **S**aline infusion, **E**ndocrine (Addison’s), **D**iarrhoea, **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, **P**araldehyde, **I**soniazid/Iron, **L**actic acidosis, **E**thylene glycol, **S**alicylates. * **Key Distinguisher:** If a patient has NAGMA and the cause isn't obvious, calculate the **Urinary Anion Gap** to differentiate between Diarrhoea (negative UAG) and RTA (positive UAG).

Q: In severe metabolic alkalosis, which of the following is NOT typically seen?

Pulmonary edema. In metabolic alkalosis, the body attempts to compensate by retaining acid through **respiratory compensation**. This involves hypoventilation to increase $PCO_2$ levels. ### Why Pulmonary Edema is the Correct Answer **Pulmonary edema** is not a feature of metabolic alkalosis. In fact, metabolic alkalosis is more commonly associated with **hypoventilation**. Pulmonary edema typically presents with respiratory distress and often leads to respiratory acidosis (due to impaired gas exchange) or is a consequence of fluid overload/heart failure, rather than a result of an alkaline pH. ### Explanation of Incorrect Options * **Hypoxia (Option B):** To compensate for the high pH, the respiratory center reduces the rate and depth of breathing (hypoventilation) to retain $CO_2$. This compensatory mechanism can lead to secondary hypoxia. * **Hypocalcemia (Option C):** Alkalosis increases the binding of ionized calcium ($Ca^{2+}$) to serum albumin. While total calcium remains normal, the **physiologically active ionized calcium decreases**, leading to functional hypocalcemia. * **Tetany (Option D):** As a direct result of the decreased ionized calcium levels, neuromuscular irritability increases, which can manifest clinically as tetany, carpopedal spasms, or positive Chvostek/Trousseau signs. ### High-Yield NEET-PG Pearls * **The "Shift" Rule:** In alkalosis, $H^+$ ions move out of cells while $K^+$ moves in, often leading to **hypokalemia**. * **Oxyhemoglobin Dissociation Curve:** Alkalosis causes a **left shift** (increased affinity of hemoglobin for $O_2$), further worsening tissue hypoxia. * **Compensation Limit:** Respiratory compensation for metabolic alkalosis is limited because the resulting hypoxia eventually stimulates the peripheral chemoreceptors to maintain a minimum level of ventilation.

Q: Metabolic acidosis is compensated by which of the following mechanisms?

Hyperventilation. **Explanation:** **1. Why Hyperventilation is Correct:** Metabolic acidosis is characterized by a primary decrease in plasma bicarbonate ($HCO_3^-$) and a drop in pH. The body employs **Respiratory Compensation** as the first line of defense. The acidic environment (high $H^+$) stimulates peripheral chemoreceptors in the carotid and aortic bodies, which signal the respiratory center in the medulla to increase the rate and depth of breathing (**Hyperventilation**). This leads to the "washout" of $CO_2$ (hypocapnia). According to the Henderson-Hasselbalch equation, reducing $PCO_2$ helps restore the $HCO_3^-/PCO_2$ ratio toward normal, thereby raising the pH. This is clinically recognized as **Kussmaul breathing**. **2. Analysis of Incorrect Options:** * **B. Bicarbonate loss:** This is a *cause* of metabolic acidosis (e.g., diarrhea or Renal Tubular Acidosis), not a compensatory mechanism. Compensation aims to conserve, not lose, bicarbonate. * **C. Chloride loss:** Chloride shifts usually occur to maintain electroneutrality but are not the primary compensatory mechanism for acidosis. * **D. Increased ammonia excretion:** While the kidney does increase $NH_4^+$ excretion to eliminate $H^+$ ions, this is a **slow process** (taking 3–5 days). In the context of "compensation" for an acute acid-base shift, respiratory hyperventilation is the immediate and primary physiological response. **High-Yield Clinical Pearls for NEET-PG:** * **Winters’ Formula:** Expected $PCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$. If the measured $PCO_2$ is higher than calculated, a concurrent respiratory acidosis exists. * **Anion Gap (AG):** Always calculate AG in metabolic acidosis ($Na^+ - [Cl^- + HCO_3^-]$). Normal is 8–12 mEq/L. * **Speed of Compensation:** Respiratory compensation starts within minutes; Renal compensation (bicarbonate regeneration) takes days.

Question 1

Given a patient with pH = 7.27, HCO3 = 10 mEq/dl, and pCO2 = 23 mm Hg, what is the acid-base disorder?

Accepted Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Question 2

Which set of arterial blood gas values describes a heavy smoker with COPD who is becoming increasingly somnolent?

Accepted Answer

pH: 7.2; HCO3-: 30 mEq/L; PCO2: 60 mmHg

Answer

pH: 7.65; HCO3-: 48 mEq/L; PCO2: 45 mmHg

Answer

pH: 7.5; HCO3-: 12 mEq/L; PCO2: 20 mmHg

Answer

pH: 7.4; HCO3-: 18 mEq/L; PCO2: 40 mmHg

Question 3

Metabolic acidosis is associated with all of the following conditions EXCEPT?

Accepted Answer

Prolonged vomiting

Answer

Chronic renal failure

Answer

Starvation

Answer

Uncontrolled Diabetes Mellitus

Question 4

Which one of the following is most likely to result in contraction alkalosis?

Accepted Answer

Loop diuretics

Answer

Laxatives

Answer

Infant formula

Answer

Acetazolamide

Question 5

Recurrent vomiting due to intestinal obstruction causes what type of acid-base disturbance?

Accepted Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Answer

Metabolic acidosis

Question 6

A normal-anion-gap metabolic acidosis occurs in patients with:

Accepted Answer

Diarrhoea

Answer

Diabetic ketoacidosis

Answer

Methyl alcohol poisoning

Answer

Acute renal failure

Question 7

Which of the following conditions can themselves cause metabolic acidosis?

Accepted Answer

Diarrhea, Ethyl alcohol, Renal tubular acidosis

Answer

Diarrhea, Diuretic, Ethyl alcohol

Answer

Diuretic, Ethyl alcohol

Answer

Diuretic, Ethyl alcohol, Renal tubular acidosis

Question 8

In severe metabolic alkalosis, which of the following is NOT typically seen?

Accepted Answer

Pulmonary edema

Answer

Hypoxia

Answer

Hypocalcemia

Answer

Tetany

Question 9

Metabolic acidosis is compensated by which of the following mechanisms?

Accepted Answer

Hyperventilation

Answer

Bicarbonate loss

Answer

Chloride loss

Answer

Increased ammonia excretion in the kidney

Question 10

Respiratory acidosis is caused by all except?

Accepted Answer

Pulmonary hypertension

Answer

Chronic bronchitis

Answer

COPD

Answer

Interstitial lung disease

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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