Acid-Base Balance — MCQs

Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 31: Metabolic acidosis is associated with all of the following conditions EXCEPT?

A. Chronic renal failure
B. Prolonged vomiting (Correct Answer)
C. Starvation
D. Uncontrolled Diabetes Mellitus

Explanation: **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.

Question 32: Which one of the following is most likely to result in contraction alkalosis?

A. Laxatives
B. Infant formula
C. Loop diuretics (Correct Answer)
D. Acetazolamide

Explanation: **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.

Question 33: Recurrent vomiting due to intestinal obstruction causes what type of acid-base disturbance?

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

Explanation: ### Explanation **Correct Option: D. Metabolic Alkalosis** **Mechanism:** Recurrent vomiting, particularly in high intestinal or gastric outlet obstruction, leads to a significant loss of gastric juice. Gastric juice is rich in **Hydrochloric acid (HCl)**. 1. **Loss of H⁺:** For every proton (H⁺) secreted into the stomach, a bicarbonate ion (HCO₃⁻) is added to the blood (the "alkaline tide"). When H⁺ is lost via vomiting, these bicarbonate ions are not neutralized, leading to an increase in plasma pH. 2. **Loss of Cl⁻:** The loss of chloride leads to **hypochloremia**. To maintain electrical neutrality, the kidneys reabsorb more bicarbonate. 3. **Volume Depletion:** Vomiting causes dehydration, activating the Renin-Angiotensin-Aldosterone System (RAAS). Aldosterone promotes Na⁺ reabsorption at the expense of H⁺ and K⁺ excretion in the distal tubule, further worsening the alkalosis (Contraction Alkalosis). **Why other options are incorrect:** * **A & B (Respiratory Acidosis/Alkalosis):** These are primary disturbances of ventilation ($PCO_2$). Vomiting is a metabolic process involving the loss of fixed acids, not a primary lung pathology. * **C (Metabolic Acidosis):** This occurs in conditions like diarrhea (loss of HCO₃⁻), diabetic ketoacidosis, or lactic acidosis. Vomiting causes the loss of acid, not base. **High-Yield Clinical Pearls for NEET-PG:** * **Paradoxical Aciduria:** In severe vomiting, despite systemic alkalosis, the urine becomes acidic. This happens because the body prioritizes Na⁺ conservation (due to dehydration) over H⁺ conservation. * **Electrolyte Triad:** Vomiting typically results in **Hypokalemic, Hypochloremic, Metabolic Alkalosis**. * **Exception:** If the obstruction is distal to the Ampulla of Vater (lower intestinal obstruction), the vomitus may contain alkaline biliary and pancreatic secretions, potentially leading to metabolic acidosis, though metabolic alkalosis remains the classic exam answer for "gastric" vomiting.

Question 34: A normal-anion-gap metabolic acidosis occurs in patients with:

A. Diarrhoea (Correct Answer)
B. Diabetic ketoacidosis
C. Methyl alcohol poisoning
D. Acute renal failure

Explanation: **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).

Question 35: Which of the following conditions can themselves cause metabolic acidosis?

A. Diarrhea, Diuretic, Ethyl alcohol
B. Diarrhea, Ethyl alcohol, Renal tubular acidosis (Correct Answer)
C. Diuretic, Ethyl alcohol
D. Diuretic, Ethyl alcohol, Renal tubular acidosis

Explanation: **Explanation:** Metabolic acidosis is characterized by a primary decrease in serum bicarbonate ($HCO_3^-$) or an accumulation of fixed acids. The correct option (B) includes three distinct mechanisms leading to this state: 1. **Diarrhea:** Intestinal secretions below the stomach are rich in bicarbonate. Profuse diarrhea leads to the direct gastrointestinal loss of $HCO_3^-$, resulting in **Normal Anion Gap Metabolic Acidosis (NAGMA)**. 2. **Ethyl Alcohol:** Metabolism of ethanol can lead to dehydration and starvation, triggering ketogenesis. Additionally, ethanol metabolism increases the NADH/NAD+ ratio, favoring the conversion of pyruvate to lactate. This results in **High Anion Gap Metabolic Acidosis (HAGMA)** due to ketoacids and lactic acid. 3. **Renal Tubular Acidosis (RTA):** This group of disorders involves either a failure to reabsorb $HCO_3^-$ (Type 2) or a failure to excrete $H^+$ (Type 1 and 4), leading to **NAGMA**. **Why other options are incorrect:** Options A, C, and D are incorrect because they include **Diuretics** (specifically loop and thiazide diuretics). Diuretics typically cause **Metabolic Alkalosis** (Contraction Alkalosis) due to the loss of chloride, hydrogen ions, and potassium, which stimulates bicarbonate reabsorption. *Note: Carbonic anhydrase inhibitors (Acetazolamide) are the exception as they cause acidosis, but "Diuretics" as a general term in exams refers to the more common loop/thiazide types.* **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, **Lactic acidosis**, **Ethylene glycol/Ethanol**, Salicylates). * **Mnemonic for NAGMA:** HARDUP (Hyperalimentation, Acetazolamide, **Renal tubular acidosis**, **Diarrhea**, Ureteroenteric fistula, Pancreatic fistula). * **Winter’s Formula:** Used to calculate expected $pCO_2$ compensation in metabolic acidosis: $pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$.

Question 36: Metabolic acidosis may result from which of the following conditions?

A. Diabetic ketoacidosis
B. Diarrhea
C. Renal failure
D. All of the above (Correct Answer)

Explanation: **Explanation:** Metabolic acidosis is characterized by a primary decrease in plasma bicarbonate ($HCO_3^-$) and a reduction in pH. This occurs through three main mechanisms: increased production of non-volatile acids, decreased renal excretion of acids, or excessive loss of bicarbonate. 1. **Diabetic Ketoacidosis (DKA):** In insulin deficiency, the body shifts to fat metabolism, producing ketoacids (acetoacetate and $\beta$-hydroxybutyrate). These are strong acids that dissociate, releasing $H^+$ ions which consume $HCO_3^-$, leading to a **High Anion Gap Metabolic Acidosis (HAGMA)**. 2. **Diarrhea:** Intestinal secretions below the stomach are rich in bicarbonate. Severe diarrhea leads to the direct physical loss of $HCO_3^-$ from the GI tract. This results in a **Normal Anion Gap (Hyperchloremic) Metabolic Acidosis**. 3. **Renal Failure:** In chronic kidney disease, the kidneys fail to excrete the daily "fixed" acid load (mainly phosphoric and sulfuric acids) and show impaired ammonia production ($NH_4^+$ excretion). This accumulation of organic anions leads to **HAGMA**. **High-Yield Clinical Pearls for NEET-PG:** * **Anion Gap (AG) Formula:** $Na^+ - (Cl^- + HCO_3^-)$. Normal range is $8–12\ mmol/L$. * **Mnemonic for HAGMA (MUDPILES):** **M**ethanol, **U**remia (Renal failure), **D**KA, **P**araldehyde, **I**soniazid/Iron, **L**actic acidosis, **E**thylene glycol, **S**alicylates. * **Normal AG Acidosis:** Primarily caused by Diarrhea, Renal Tubular Acidosis (RTA), and Acetazolamide use. * **Winter’s Formula:** Used to calculate expected $pCO_2$ compensation: $1.5 \times [HCO_3^-] + 8 \pm 2$. If the measured $pCO_2$ differs, a mixed acid-base disorder is present.

Question 37: Which protein is the major binder of thyroxine?

A. Albumin
B. Prealbumin
C. Globulin (Correct Answer)
D. Transferrin

Explanation: **Explanation:** Thyroid hormones (T3 and T4) are highly lipophilic and require carrier proteins for transport in the blood. While several proteins bind thyroxine, **Thyroxine-Binding Globulin (TBG)** is the most significant. **1. Why Globulin is Correct:** TBG is a glycoprotein synthesized in the liver. Although it is present in much lower concentrations than albumin, it has an extremely **high affinity** for T4. Consequently, it carries approximately **70% of the total circulating T4**. This high-affinity binding ensures a stable reservoir of the hormone and prevents its rapid excretion. **2. Analysis of Incorrect Options:** * **Albumin (A):** While albumin has the highest *capacity* to bind T4 due to its high plasma concentration, it has a very **low affinity**. It carries only about 10-15% of circulating T4. * **Prealbumin (B):** Also known as **Transthyretin (TTR)**, it carries about 10-15% of T4. It is more important for T4 transport into the cerebrospinal fluid (CSF) than in general systemic circulation. * **Transferrin (D):** This is the primary transport protein for **Iron**, not thyroid hormones. **Clinical Pearls for NEET-PG:** * **Free Hormone Hypothesis:** Only the unbound (free) T3 and T4 are biologically active. * **TBG Levels:** TBG increases in **high-estrogen states** (Pregnancy, OCP use), leading to increased *Total* T4, but *Free* T4 remains normal (Euthyroid). * **TBG Decreases:** Seen in **Nephrotic syndrome**, liver failure, and with androgen use. * **Binding Affinity Order:** TBG > Prealbumin > Albumin.

Question 38: In severe metabolic alkalosis, which of the following is NOT typically seen?

A. Pulmonary edema (Correct Answer)
B. Hypoxia
C. Hypocalcemia
D. Tetany

Explanation: 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.

Question 39: Metabolic acidosis is compensated by which of the following mechanisms?

A. Hyperventilation (Correct Answer)
B. Bicarbonate loss
C. Chloride loss
D. Increased ammonia excretion in the kidney

Explanation: **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 40: What is the pH of blood under normal physiological conditions?

A. 6.8
B. 7.1
C. 7.4 (Correct Answer)
D. 7.9

Explanation: **Explanation:** The normal physiological pH of arterial blood is strictly maintained between **7.35 and 7.45**, with the average value being **7.4**. This slightly alkaline state is essential for optimal enzymatic activity, protein structure, and cellular metabolism. The body utilizes three primary systems to maintain this narrow range: chemical buffers (like the Bicarbonate-Carbonic acid system), the respiratory system (CO₂ regulation), and the renal system (H⁺ excretion and HCO₃⁻ reabsorption). **Analysis of Options:** * **Option C (7.4):** This is the correct physiological midpoint. At this pH, the ratio of bicarbonate (HCO₃⁻) to dissolved CO₂ is approximately **20:1**, as described by the Henderson-Hasselbalch equation. * **Option A (6.8) & B (7.1):** These represent states of severe **Acidemia**. A pH of 6.8 is generally considered the lower limit of life; values below this lead to fatal cardiac arrhythmias and CNS depression. * **Option D (7.9):** This represents severe **Alkalemia**. A pH above 7.8 is typically incompatible with life, leading to severe neuromuscular excitability and tetany. **NEET-PG Clinical Pearls:** * **Venous Blood pH:** Slightly more acidic than arterial blood (approx. **7.35**) due to the higher concentration of CO₂ (forming carbonic acid). * **Intracellular pH:** Usually lower than plasma pH, ranging from **6.0 to 7.4** depending on the metabolic activity of the cell. * **Survival Range:** The human body can only tolerate a blood pH range of approximately **6.8 to 8.0** for short periods. * **Anion Gap:** Always calculate the Anion Gap $[Na^+ - (Cl^- + HCO_3^-)]$ when evaluating metabolic acidosis; the normal range is **8–12 mEq/L**.

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Acid-Base Chemistry

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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

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Respiratory Acidosis and Alkalosis

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Metabolic Acidosis and Alkalosis

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Mixed Acid-Base Disorders

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Compensatory Mechanisms

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Clinical Assessment of Acid-Base Status

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