Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance Indian Medical PG Practice Questions and MCQs

Question 61: Normal anionic gap is seen in which of the following conditions?

A. Diarrhea (Correct Answer)
B. Uremia
C. Lactic acidosis
D. Ketosis

Explanation: ### Explanation **Concept Overview** The Anion Gap (AG) is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. In metabolic acidosis, the gap depends on whether the loss of bicarbonate is replaced by chloride (Normal AG) or by an unmeasured acid anion (High AG). **1. Why Diarrhea is Correct (Normal Anion Gap Metabolic Acidosis - NAGMA)** In **Diarrhea**, there is a direct gastrointestinal loss of bicarbonate ($HCO_3^-$). To maintain electroneutrality, the kidneys retain Chloride ($Cl^-$). Because the decrease in bicarbonate is exactly offset by an increase in chloride, the Anion Gap remains within the normal range (8–12 mEq/L). This is also known as **Hyperchloremic Metabolic Acidosis**. **2. Why the Other Options are Incorrect (High Anion Gap Metabolic Acidosis - HAGMA)** In these conditions, bicarbonate is consumed to buffer an "unmeasured" organic acid. Since chloride levels do not rise to compensate, the gap increases: * **Uremia:** Failure to excrete fixed acids (phosphates, sulfates) leads to their accumulation. * **Lactic Acidosis:** Accumulation of lactate (e.g., in shock or hypoxia). * **Ketosis:** Accumulation of acetoacetate and beta-hydroxybutyrate (e.g., Diabetic Ketoacidosis). **NEET-PG High-Yield Pearls** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, Infection/Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA:** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra-chloride, Diarrhea, Carbonic anhydrase inhibitors, Adrenal insufficiency, Renal Tubular Acidosis, Pancreatic fistula). * **Key Distinction:** If the question mentions **Renal Tubular Acidosis (RTA)**, it is a classic cause of NAGMA, frequently tested alongside diarrhea.

Question 62: Metabolic alkalosis is caused by:

A. Repeated vomiting (Correct Answer)
B. Diarrhea
C. Diabetic ketosis
D. All of the above

Explanation: **Explanation:** **Metabolic alkalosis** is characterized by a primary increase in serum bicarbonate ($HCO_3^-$) and an increase in blood pH (>7.45). **1. Why Repeated Vomiting is Correct:** Gastric juice is highly acidic, containing high concentrations of hydrochloric acid ($HCl$). During **repeated vomiting** (or gastric suctioning), there is a massive loss of hydrogen ions ($H^+$) and chloride ions ($Cl^-$). As $H^+$ is lost, the parietal cells of the stomach generate more $HCO_3^-$ which enters the bloodstream (the "alkaline tide"). Furthermore, the resulting volume depletion triggers the Renin-Angiotensin-Aldosterone System (RAAS), leading to further $H^+$ excretion in the kidneys to conserve sodium, maintaining the alkalotic state. **2. Why Incorrect Options are Wrong:** * **Diarrhea:** Intestinal secretions are rich in bicarbonate. Loss of these fluids leads to a net loss of base, resulting in **Normal Anion Gap Metabolic Acidosis**. * **Diabetic Ketosis (DKA):** This condition involves the overproduction of organic acids (acetoacetate and $\beta$-hydroxybutyrate). The accumulation of these fixed acids consumes bicarbonate, leading to a **High Anion Gap Metabolic Acidosis**. **High-Yield Clinical Pearls for NEET-PG:** * **Saline Responsiveness:** Vomiting-induced alkalosis is "Saline Responsive" (Urinary $Cl^-$ < 10 mmol/L) because chloride replacement helps the kidneys excrete excess bicarbonate. * **Hypokalemia:** Metabolic alkalosis is almost always associated with hypokalemia, as $K^+$ shifts intracellularly in exchange for $H^+$ to buffer the serum pH. * **Paradoxical Aciduria:** In severe dehydration due to vomiting, the kidney prioritizes sodium reabsorption over pH balance, excreting $H^+$ instead of $Na^+$, leading to acidic urine despite systemic alkalosis.

Question 63: Which of the following statements regarding high anion gap is FALSE?

A. Seen in uremia
B. Iron causes high anion gap
C. Hyperchloremia is present (Correct Answer)
D. Acidosis is present

Explanation: **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. In a normal state, the gap (representing unmeasured anions like albumin and phosphates) is 8–12 mEq/L. **Why Option C is the correct (False) statement:** In **High Anion Gap Metabolic Acidosis (HAGMA)**, the acidosis is caused by the accumulation of "unmeasured" organic acids (e.g., lactate, ketones). As these acids dissociate, the $H^+$ ions consume $HCO_3^-$ (buffering), while the acid anions take the place of the lost bicarbonate to maintain electroneutrality. Therefore, the **Chloride levels remain normal**. In contrast, **Hyperchloremia** is the hallmark of **Normal Anion Gap Metabolic Acidosis (NAGMA)**, where the loss of $HCO_3^-$ is directly compensated by an increase in $Cl^-$ to maintain balance. **Analysis of Incorrect Options:** * **A. Seen in uremia:** True. In renal failure, the kidneys fail to excrete fixed acids like phosphates and sulfates, which act as unmeasured anions, increasing the AG. * **B. Iron causes high anion gap:** True. Iron toxicity causes mitochondrial dysfunction leading to severe **Lactic Acidosis**, a classic cause of HAGMA. * **D. Acidosis is present:** True. By definition, a high anion gap in this context refers to metabolic acidosis where bicarbonate is consumed. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA (Hyperchloremic):** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhea, Carbonic anhydrase inhibitors, Renal Tubular Acidosis, Pancreatic fistula). * **Gold Standard:** If the question mentions diarrhea or RTA, always look for **Hyperchloremia/NAGMA**. If it mentions shock, toxins, or renal failure, think **HAGMA**.

Question 64: Which of the following is not a cause of metabolic alkalosis?

A. Vomiting
B. Fever
C. Renal failure
D. Both vomiting and renal failure (Correct Answer)

Explanation: **Explanation:** Metabolic alkalosis is characterized by an increase in plasma bicarbonate ($HCO_3^-$) and a rise in arterial pH (>7.45). To answer this question, we must distinguish between causes of alkalosis and acidosis. **1. Why "Both Vomiting and Renal Failure" is the correct choice (in the context of the question's logic):** * **Renal Failure:** This is a classic cause of **High Anion Gap Metabolic Acidosis (HAGMA)**. In renal failure, the kidneys fail to excrete fixed acids (phosphates, sulfates) and cannot effectively regenerate bicarbonate. Therefore, it causes acidosis, not alkalosis. * **Fever:** Fever increases the metabolic rate, leading to increased $CO_2$ production and often compensatory hyperventilation. This typically results in **Respiratory Alkalosis**, not metabolic alkalosis. * Since both B and C do *not* cause metabolic alkalosis, the question structure identifies "Both" as the non-causative factors (Note: While the option says "Vomiting and Renal Failure," in standard medical exams, if multiple options are incorrect, the "Both" option usually points to the non-alkalotic states). **2. Analysis of Options:** * **Vomiting (Incorrect as a "not" cause):** Gastric juice is rich in $HCl$. Loss of stomach acid via vomiting leads to a loss of hydrogen ions and a relative gain of bicarbonate (the "alkaline tide"), causing **Metabolic Alkalosis**. * **Fever:** As stated, this leads to respiratory alkalosis due to tachypnea. * **Renal Failure:** Leads to metabolic acidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Metabolic Alkalosis:** "CLEVER" (Chloride depletion, Licorice, Endocrine/Conn’s, Vomiting, Excess Alkali, Renal/Bartter’s & Gitelman’s). * **Saline Responsiveness:** Vomiting-induced alkalosis is "Saline Responsive" (Urinary $Cl^-$ < 10 mEq/L), whereas Mineralocorticoid excess is "Saline Resistant" (Urinary $Cl^-$ > 20 mEq/L). * **Renal Failure Mnemonic:** Part of **MUDPILES** for HAGMA (U = Uremia/Renal Failure).

Question 65: Shohl's solution is used for the treatment of?

A. Hypokalemia
B. Hyperkalemia
C. Metabolic acidosis (Correct Answer)
D. Hungry bones syndrome

Explanation: **Explanation:** **Shohl’s solution** is an oral alkalinizing agent consisting of **sodium citrate and citric acid**. **1. Why Metabolic Acidosis is the Correct Answer:** The primary mechanism involves the metabolism of citrate. Once ingested, citrate is metabolized in the liver to form **bicarbonate (HCO₃⁻)**. This increase in systemic bicarbonate helps neutralize excess hydrogen ions, making it a mainstay in the management of **Distal Renal Tubular Acidosis (Type 1 RTA)** and chronic metabolic acidosis associated with renal failure. It is also used to alkalinize urine to prevent uric acid and cystine stones. **2. Analysis of Incorrect Options:** * **Hypokalemia (A):** Shohl’s solution is sodium-based. While it doesn't treat hypokalemia, a similar preparation called **Bicitra** or **Polycitra** (containing potassium citrate) might be used if potassium supplementation is also needed. * **Hyperkalemia (B):** Shohl’s solution does not lower serum potassium levels. In fact, in patients with severe renal failure, sodium-based alkalinizing agents must be used cautiously to avoid fluid overload. * **Hungry Bone Syndrome (D):** This condition occurs post-parathyroidectomy, characterized by profound hypocalcemia. Treatment requires aggressive calcium and Vitamin D supplementation, not citrate buffers. **3. High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Sodium citrate (100 mg/ml) + Citric acid (67 mg/ml). * **Citrate vs. Bicarbonate:** Citrate is often preferred over oral sodium bicarbonate because it is more palatable and produces less gastric distension (no CO₂ gas release in the stomach). * **Aluminum Toxicity Warning:** Citrate significantly **increases the absorption of aluminum** from the gut. Therefore, Shohl’s solution should never be co-administered with aluminum-containing antacids (common in CKD patients), as it can lead to aluminum neurotoxicity.

Question 66: Which of the following amino acids, responsible for the buffering action of hemoglobin, is also a major intracellular buffer of blood?

A. Histidine (Correct Answer)
B. Arginine
C. Valine
D. Lysine

Explanation: **Explanation:** The buffering capacity of a protein is primarily determined by the **pKa of the side chains** of its constituent amino acids. For an amino acid to be an effective buffer at physiological pH (~7.4), its pKa must be close to that value. **Why Histidine is correct:** Histidine contains an **imidazole side chain** with a pKa of approximately **6.0 to 6.1**. While this is slightly below physiological pH, it is the closest among all amino acids. In hemoglobin, the local environment of the protein shifts the pKa of certain histidine residues (especially the C-terminal histidine) closer to 7.0, making it the most efficient buffer in the blood. Hemoglobin contains 38 histidine residues, allowing it to account for the majority of the non-bicarbonate buffering capacity of whole blood. **Why other options are incorrect:** * **Arginine (pKa ~12.5) and Lysine (pKa ~10.5):** These are basic amino acids. Their side chains are almost entirely protonated at physiological pH, meaning they cannot effectively donate or accept protons to buffer changes around pH 7.4. * **Valine:** This is a non-polar, branched-chain amino acid with no ionizable side chain. It cannot participate in acid-base buffering. **High-Yield Clinical Pearls for NEET-PG:** * **Bohr Effect:** Histidine residues in hemoglobin (specifically His-146) play a crucial role in the Bohr effect by binding H+ ions, which stabilizes the T-state (deoxyhemoglobin) and promotes oxygen release in tissues. * **Intracellular vs. Extracellular:** While **Bicarbonate** is the major *extracellular* buffer, **Proteins (Histidine)** and **Phosphates** are the major *intracellular* buffers. * **Maximum Buffering:** A buffer is most effective when the pH of the solution is equal to the pKa of the buffer (pH = pKa).

Question 67: Which is the most important buffer in the extracellular fluid?

A. Phosphate
B. Acetate
C. Bicarbonate (Correct Answer)
D. Plasma protein

Explanation: **Explanation:** The **Bicarbonate-Carbonic Acid system** is the most important buffer in the Extracellular Fluid (ECF) for two primary reasons: its high concentration and its status as an **"open system."** Unlike other buffers, its components are independently regulated by two major organs: the **lungs** (which control $CO_2$ via respiration) and the **kidneys** (which regulate $HCO_3^-$ excretion and regeneration). This allows the body to maintain the physiological pH of 7.4 despite constant metabolic acid production. **Analysis of Options:** * **Phosphate (Option A):** While it has a pKa (6.8) closer to physiological pH than bicarbonate, its concentration in the ECF is too low to be the primary buffer. It is, however, the **most important internal tubular buffer** in the kidneys and a major intracellular buffer. * **Acetate (Option B):** This is a metabolic intermediate and not a physiological buffer system used to maintain ECF pH. * **Plasma Proteins (Option D):** Proteins (like albumin) act as buffers due to their imidazole groups (histidine). While significant, they are secondary to the bicarbonate system in the ECF. **High-Yield Clinical Pearls for NEET-PG:** * **Henderson-Hasselbalch Equation:** $pH = pKa + \log([HCO_3^-] / [0.03 \times PCO_2])$. * **Most important intracellular buffer:** Proteins and Phosphate. * **Most important buffer in RBCs:** Hemoglobin (due to the Bohr effect and carbamino compounds). * **First line of defense** against pH shift: Chemical buffers (seconds); **Second line:** Respiratory system (minutes); **Third line:** Renal system (hours to days).

Question 68: A 30-year-old female presents with low serum calcium and phosphate levels, along with elevated parathormone. What is the most likely diagnosis?

A. Vitamin D deficiency (Correct Answer)
B. Primary hyperparathyroidism
C. Osteoporosis
D. Paget's disease

Explanation: ### Explanation The biochemical profile of **low serum calcium**, **low serum phosphate**, and **elevated Parathyroid Hormone (PTH)** is classic for **Vitamin D deficiency** (Osteomalacia in adults). **1. Why Vitamin D Deficiency is Correct:** Vitamin D is essential for the intestinal absorption of both calcium and phosphate. A deficiency leads to decreased levels of both minerals. In response to low serum calcium (hypocalcemia), the parathyroid glands increase the secretion of PTH (**Secondary Hyperparathyroidism**). While PTH helps mobilize calcium from bones to normalize serum levels, it also increases phosphate excretion in the urine (phosphaturia), further lowering serum phosphate levels. **2. Why the Other Options are Incorrect:** * **Primary Hyperparathyroidism:** Characterized by **elevated** serum calcium and low phosphate due to autonomous PTH secretion (usually a parathyroid adenoma). * **Osteoporosis:** Typically presents with **normal** serum calcium, phosphate, and PTH levels. It is a quantitative decrease in bone mass, not a mineral metabolism defect. * **Paget’s Disease:** Usually shows **normal** calcium and phosphate levels, though Alkaline Phosphatase (ALP) is significantly elevated. Calcium may only rise during periods of prolonged immobilization. **3. NEET-PG High-Yield Pearls:** * **Secondary Hyperparathyroidism:** PTH is high as a *reaction* to low calcium (e.g., Vitamin D deficiency, Chronic Kidney Disease). * **Pseudohypoparathyroidism:** Presents with low calcium and high phosphate, but PTH is **high** due to end-organ resistance to PTH. * **Hypoparathyroidism:** Both calcium and PTH are **low**, while phosphate is high. * **Key Marker:** In Vitamin D deficiency, **Alkaline Phosphatase (ALP)** is typically elevated due to increased osteoblastic activity attempting to mineralize bone.

Question 69: Anion gap is mostly due to:

A. Proteins (Correct Answer)
B. Sulphates
C. Phosphates
D. Nitrates

Explanation: **Explanation:** The **Anion Gap (AG)** is a calculated value representing the difference between measured cations (Sodium) and measured anions (Chloride and Bicarbonate). Since the body must maintain electrical neutrality, the "gap" represents unmeasured anions present in the plasma. **1. Why Proteins are the Correct Answer:** Under physiological conditions, plasma proteins—primarily **Albumin**—carry a significant net negative charge due to the dissociation of carboxyl groups at a pH of 7.4. Albumin is the single largest contributor to the normal anion gap, accounting for approximately **75-80%** of its value (roughly 1.5 to 2.5 mEq/L for every 1 g/dL of albumin). **2. Analysis of Incorrect Options:** * **Sulphates and Phosphates (B & C):** While these are indeed "unmeasured anions" that contribute to the anion gap, their concentrations in healthy individuals are significantly lower than that of plasma proteins. They become clinically significant primarily in cases of renal failure (Uremia). * **Nitrates (D):** Nitrates are not standard physiological components of the plasma anion gap calculation and do not contribute significantly to its value. **3. Clinical Pearls for NEET-PG:** * **Normal Range:** 8–12 mEq/L (or 10–14 mEq/L depending on the lab). * **Formula:** $AG = Na^+ - (Cl^- + HCO_3^-)$. * **The Albumin Factor:** In patients with **Hypoalbuminemia**, the "normal" anion gap is lower. For every 1 g/dL decrease in serum albumin, the anion gap decreases by approximately **2.5 mEq/L**. Failure to adjust for this can mask a High Anion Gap Metabolic Acidosis (HAGMA). * **Mnemonic for HAGMA:** MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates).

Question 70: The normal anion gap is primarily attributed to which of the following?

A. Unmeasured cations
B. Unmeasured anions
C. Unmeasured proteins (Correct Answer)
D. None of the above

Explanation: **Explanation:** The **Anion Gap (AG)** is a calculated value used to identify the cause of metabolic acidosis. It represents the difference between measured cations (Sodium) and measured anions (Chloride and Bicarbonate). **1. Why the Correct Answer is Right:** According to the principle of electroneutrality, the total number of positive charges must equal the total number of negative charges in the serum. However, in routine clinical practice, we only measure a few ions. * **Formula:** $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$ * The "gap" exists because there are more unmeasured anions than unmeasured cations. Among these unmeasured anions, **Albumin (Serum Protein)** is the most significant contributor. Albumin carries a strong negative charge at physiological pH, accounting for nearly **75-80%** of the normal anion gap (approximately 12 ± 2 mEq/L). **2. Why Other Options are Wrong:** * **Option A (Unmeasured Cations):** These include Calcium, Magnesium, and Potassium. If unmeasured cations were the primary cause, the gap would be negative or zero, as they would balance the measured anions. * **Option B (Unmeasured Anions):** While this is technically true (the gap *is* the sum of unmeasured anions like phosphates, sulfates, and organic acids), **Option C** is the most specific and correct answer for a *normal* state. In a healthy individual, proteins (Albumin) are the dominant component of these unmeasured anions. **3. High-Yield Clinical Pearls for NEET-PG:** * **Correction for Albumin:** For every **1 g/dL decrease** in serum albumin below the normal (4 g/dL), the observed Anion Gap decreases by approximately **2.5 mEq/L**. * **Hypoalbuminemia:** This is the most common cause of a **low anion gap**. * **MUDPILES:** This mnemonic represents causes of High Anion Gap Metabolic Acidosis (HAGMA), where unmeasured anions like Lactate or Ketoacids increase.

Practice by Chapter

Acid-Base Chemistry and Buffers

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pH Regulation in Body Fluids

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Respiratory Regulation of Acid-Base Balance

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Renal Regulation of Acid-Base Balance

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Respiratory and Metabolic Acidosis

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Respiratory and Metabolic Alkalosis

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Mixed Acid-Base Disorders

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Interpretation of Arterial Blood Gases

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Electrolyte Homeostasis

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Sodium and Water Balance

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Potassium Balance

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Calcium and Phosphate Metabolism

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