Acid-Base and Electrolyte Balance — MCQs

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

Question 151: Histidine is the most important amino acid for buffering under normal physiological conditions because?

A. It regulates kidney acid-base balance
B. Its pKa value is higher than physiological pH
C. Its pKa value is closer to physiological pH (Correct Answer)
D. Its pKa value is very low

Explanation: ### Explanation **1. Why Option C is Correct:** The buffering capacity of any substance is maximal when the pH of the environment is close to its **pKa value** (the pH at which 50% of the molecule is ionized and 50% is unionized). According to the Henderson-Hasselbalch equation, a buffer is most effective within **±1 pH unit** of its pKa. * The physiological pH of human blood is approximately **7.4**. * The pKa of the imidazole side chain of **Histidine** is approximately **6.0 to 6.1**. Among all the amino acids found in proteins, Histidine is the only one with a pKa near the physiological range. This allows it to effectively donate or accept protons at body pH, making it the primary contributor to the buffering capacity of proteins like **Hemoglobin**. **2. Why Other Options are Incorrect:** * **Option A:** While the kidneys regulate acid-base balance (via bicarbonate reabsorption and H+ secretion), this is a systemic physiological process, not the biochemical reason why a specific amino acid acts as a buffer. * **Option B:** If the pKa were significantly higher than 7.4 (like Lysine, pKa ~10.5), the amino acid would remain protonated and unable to effectively neutralize added bases at physiological pH. * **Option D:** If the pKa were very low (like Aspartic acid, pKa ~3.9), the amino acid would remain deprotonated and unable to neutralize added acids at physiological pH. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Hemoglobin (Hb) as a Buffer:** Hb is a rich source of Histidine residues (38 residues per molecule). This makes Hb the most important non-bicarbonate buffer in the blood. * **Bohr Effect:** The buffering action of Histidine in Hb is crucial for the transport of CO2 and the release of Oxygen in tissues. * **Intracellular Buffering:** Proteins (due to Histidine) are the most abundant **intracellular** buffers. * **Maximum Buffering Capacity:** Always occurs when **pH = pKa**.

Question 152: Which of the following is associated with elevated alkaline phosphatase, low calcium, and low phosphate?

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

Explanation: ### Explanation **Correct Answer: D. Vitamin D deficiency** **Mechanism:** Vitamin D is essential for the intestinal absorption of **calcium** and **phosphate**. In Vitamin D deficiency (Rickets in children, Osteomalacia in adults), the lack of absorption leads to **hypocalcemia** and **hypophosphatemia**. Low serum calcium triggers a compensatory rise in **Parathyroid Hormone (PTH)**—a condition known as secondary hyperparathyroidism. PTH attempts to restore calcium levels by mobilizing it from the bone, which increases osteoblastic activity. This increased bone turnover results in the elevation of **Alkaline Phosphatase (ALP)**, a marker of osteoblast activity. **Analysis of Incorrect Options:** * **A. Paget’s Disease:** Characterized by isolated, markedly **elevated ALP** due to excessive bone remodeling. However, serum calcium and phosphate levels are typically **normal**. * **B. Osteoporosis:** This is a quantitative decrease in bone mass. Crucially, all biochemical markers—**Calcium, Phosphate, and ALP—remain normal**. * **C. Primary Hyperparathyroidism:** Caused by autonomous PTH secretion (usually an adenoma). This leads to **hypercalcemia** and **hypophosphatemia** (due to renal phosphate wasting). ALP may be elevated only if significant bone involvement (Osteitis fibrosa cystica) is present. **High-Yield Clinical Pearls for NEET-PG:** * **ALP** is a marker of **osteoblastic** activity, while **Urinary Hydroxyproline** or **Serum NTx/CTx** are markers of **osteoclastic** activity. * **Vitamin D Deficiency Triad:** ↓ Ca²⁺, ↓ PO₄³⁻, ↑ ALP, and ↑ PTH. * **Renal Osteodystrophy:** Characterized by **Hypocalcemia** and **Hyperphosphatemia** (due to decreased renal excretion of phosphate), which distinguishes it from Vitamin D deficiency. * **Hungry Bone Syndrome:** Post-parathyroidectomy, patients show low Ca, low PO₄, and high ALP (similar to Vitamin D deficiency) due to rapid bone remineralization.

Question 153: Which of the following conditions is NOT associated with an increased anion-gap metabolic acidosis?

A. Shock
B. Ingestion of antifreeze
C. Diabetic ketoacidosis
D. COPD (Correct Answer)

Explanation: **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. An increased anion gap (HAGMA) occurs when unmeasured acid anions (like lactate, ketones, or exogenous toxins) accumulate in the blood, consuming bicarbonate. **Why COPD is the correct answer:** COPD (Chronic Obstructive Pulmonary Disease) causes **Respiratory Acidosis**, not metabolic acidosis. In COPD, the primary pathology is the failure of the lungs to eliminate $CO_2$, leading to hypercapnia (increased $pCO_2$). While the body may compensate by increasing bicarbonate levels, it does not involve the accumulation of unmeasured acid anions that characterize a high anion gap. **Analysis of Incorrect Options:** * **Shock (Option A):** Leads to tissue hypoxia and anaerobic metabolism, resulting in **Lactic Acidosis**. Lactate is an unmeasured anion that increases the AG. * **Ingestion of Antifreeze (Option B):** Antifreeze contains **Ethylene Glycol**, which is metabolized into glycolic and oxalic acids. These organic acids increase the AG. * **Diabetic Ketoacidosis (Option C):** Results in the overproduction of **$\beta$-hydroxybutyrate and acetoacetate**. These ketoacids consume bicarbonate and increase the AG. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Normal Anion Gap Acidosis (NAGMA):** Primarily caused by GI loss of $HCO_3^-$ (Diarrhea) or Renal Tubular Acidosis (RTA). * **Normal AG range:** 8–12 mEq/L. * In **COPD**, the compensation is renal (retention of $HCO_3^-$), which takes 3–5 days to fully manifest.

Question 154: Which of the following buffer systems is most effective for maintaining a physiological pH of 7.4?

A. Carbonic acid buffer (pKa = 6.1)
B. Phosphate buffer (pKa = 6.9) (Correct Answer)
C. Glutamate buffer (pKa = 8.7)
D. Acetate buffer (pKa = 4.5)

Explanation: The effectiveness of a buffer system is determined by the **Henderson-Hasselbalch equation**. A buffer is most efficient when its **pKa is closest to the desired pH** of the solution (ideally within ±1 pH unit). At this point, the concentrations of the conjugate base and weak acid are nearly equal, allowing the system to neutralize both added acids and bases effectively. ### Why Phosphate Buffer is Correct The physiological pH of blood is **7.4**. Among the options, the **Phosphate buffer (pKa = 6.9)** has a pKa closest to 7.4. While it is a minor buffer in the plasma due to low concentration, it is the **most potent intracellular buffer** and the primary buffer in **urine**, where the pH is closer to its pKa. ### Analysis of Incorrect Options * **Carbonic acid buffer (pKa = 6.1):** Although its pKa is 1.3 units away from 7.4, it is the most important **extracellular** buffer. Its effectiveness stems not from its pKa, but from being an **"open system"** where $CO_2$ levels can be rapidly adjusted by the lungs and $HCO_3^-$ by the kidneys. * **Glutamate buffer (pKa = 8.7):** The pKa is too high; it would be most effective at a basic pH of 8.7. * **Acetate buffer (pKa = 4.5):** The pKa is too low; it is effective only in highly acidic environments. ### NEET-PG High-Yield Pearls * **Maximum Buffering Capacity:** Occurs when **pH = pKa**. * **Primary Extracellular Buffer:** Bicarbonate/Carbonic acid system. * **Primary Intracellular Buffer:** Proteins (specifically **Histidine** residues due to a pKa of ~6.0) and Phosphate. * **Isohydric Principle:** All buffer systems in a common solution (like plasma) are in equilibrium with the same $[H^+]$; a change in one system affects all others.

Question 155: All are causes of increased anion gap except?

A. Diabetic ketoacidosis
B. Starvation
C. Ethylene glycol poisoning
D. Glue sniffing (Correct Answer)

Explanation: To solve this question, one must distinguish between **High Anion Gap Metabolic Acidosis (HAGMA)** and **Normal Anion Gap Metabolic Acidosis (NAGMA)**. ### **Explanation** The Anion Gap (AG) is calculated as: $[Na^+] - ([Cl^-] + [HCO_3^-])$. A high anion gap occurs when unmeasured anions (like lactate, ketones, or exogenous toxins) accumulate in the blood. **Why "Glue Sniffing" is the correct answer:** Glue sniffing involves the inhalation of **Toluene**. Toluene is metabolized to hippuric acid, which is rapidly excreted by the kidneys. This process leads to a loss of bicarbonate and a compensatory increase in chloride, resulting in **Normal Anion Gap Metabolic Acidosis (NAGMA)**. Additionally, toluene can cause Type 1 Renal Tubular Acidosis (RTA), which is a classic cause of NAGMA. **Why the other options are incorrect:** * **Diabetic Ketoacidosis (DKA):** Accumulation of acetoacetate and beta-hydroxybutyrate (unmeasured anions) leads to HAGMA. * **Starvation:** Prolonged fasting leads to the production of ketone bodies, causing HAGMA. * **Ethylene Glycol Poisoning:** Metabolism of ethylene glycol produces glycolic and oxalic acids, which are unmeasured anions that significantly increase the anion gap. ### **High-Yield Clinical Pearls for NEET-PG** * **Mnemonic for HAGMA (MUDPILES):** **M**ethanol, **U**remia, **D**KA, **P**araldehyde, **I**soniazid/Iron, **L**actic acidosis, **E**thylene glycol, **S**alicylates. * **Mnemonic for NAGMA (HARDUP):** **H**yperalimentation, **A**cetazolamide, **R**enal Tubular Acidosis (RTA), **D**iarrhea, **U**retero-sigmoidostomy, **P**ancreatic fistula. * **Key Distinction:** If a patient presents with metabolic acidosis and a **high osmolar gap**, suspect Methanol or Ethylene glycol poisoning.

Question 156: Which of the following is a precursor of the molecule shown in the illustration?

A. Insulin (Correct Answer)
B. Glucagon
C. Somatostatin
D. Lipase

Explanation: ***Insulin*** - The molecule shown is **C-peptide**, which is produced during **insulin biosynthesis** when **proinsulin** is cleaved to form insulin and C-peptide. - **Insulin** serves as the direct precursor in this pathway: **Preproinsulin** → **Proinsulin** → **Insulin + C-peptide**, making insulin the correct answer. *Glucagon* - **Glucagon** is synthesized from **proglucagon** through a completely separate biosynthetic pathway in pancreatic **alpha cells**. - It has **opposing metabolic effects** to insulin and does not contribute to **C-peptide formation**. *Somatostatin* - **Somatostatin** is derived from **prosomatostatin** in pancreatic **delta cells** and hypothalamic neurons. - It functions as a **growth hormone inhibitor** and has no role in the **insulin-C-peptide** biosynthetic pathway. *Lipase* - **Pancreatic lipase** is a **digestive enzyme** synthesized as **prolipase** and activated in the small intestine. - It is involved in **fat digestion**, not hormone biosynthesis, and has no connection to **C-peptide formation**.

Question 157: Which of the following conditions is associated with normal anion gap acidosis?

A. Lactic acidosis
B. Ketoacidosis
C. Methanol poisoning
D. Hyperchloremic acidosis (Correct Answer)

Explanation: **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as: $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$. The normal range is 8–12 mEq/L. **Why Hyperchloremic Acidosis is correct:** In **Normal Anion Gap Metabolic Acidosis (NAGMA)**, the loss of bicarbonate ($HCO_3^-$) is balanced by a reciprocal increase in serum chloride ($Cl^-$) to maintain electroneutrality. Because the sum of chloride and bicarbonate remains constant, the anion gap does not change. This is why NAGMA is synonymous with **Hyperchloremic Acidosis**. Common causes include diarrhea (GI loss of $HCO_3^-$) and Renal Tubular Acidosis (RTA). **Why the other options are incorrect:** * **Lactic Acidosis (A), Ketoacidosis (B), and Methanol Poisoning (C):** These are all causes of **High Anion Gap Metabolic Acidosis (HAGMA)**. In these conditions, metabolic acids (lactate, ketones, or formic acid) dissociate, adding "unmeasured anions" to the blood. These anions replace bicarbonate without increasing chloride, thus widening the gap. **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:** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhea, Carbonic anhydrase inhibitors, Adrenal insufficiency, Renal tubular acidosis, Pancreatic fistula). * **Key Distinction:** Diarrhea is the most common cause of NAGMA, while DKA and Lactic acidosis are the most common causes of HAGMA.

Question 158: Increased anion gap is not seen in which of the following conditions?

A. Salicylate poisoning
B. Renal tubular acidosis (Correct Answer)
C. Lactic acidosis
D. Ethylene glycol poisoning

Explanation: To understand this question, we must first define the **Anion Gap (AG)**, calculated as: $[Na^+] - ([Cl^-] + [HCO_3^-])$. A normal anion gap is typically **8–12 mEq/L**. ### Why Renal Tubular Acidosis (RTA) is the Correct Answer Metabolic acidosis is classified into two types based on the anion gap: 1. **High Anion Gap Metabolic Acidosis (HAGMA):** Occurs when fixed acids (like lactate or ketones) are added to the blood. The $HCO_3^-$ buffers these acids, but the unmeasured acid anions increase the gap. 2. **Normal Anion Gap Metabolic Acidosis (NAGMA):** Also known as **Hyperchloremic Metabolic Acidosis**. Here, the loss of $HCO_3^-$ is compensated by a proportional increase in $Cl^-$ to maintain electroneutrality. **Renal Tubular Acidosis (RTA)** is a classic cause of NAGMA because the primary defect is the inability to secrete $H^+$ or reabsorb $HCO_3^-$, leading to chloride retention. ### Analysis of Incorrect Options (Causes of HAGMA) * **Salicylate Poisoning:** Aspirin overdose leads to the accumulation of salicylic acid and interferes with the Krebs cycle, producing organic acids. * **Lactic Acidosis:** Seen in shock or hypoxia; the accumulation of lactate (an unmeasured anion) increases the gap. * **Ethylene Glycol Poisoning:** Metabolism of this antifreeze agent produces glycolic and oxalic acids, significantly raising the anion gap. ### 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 (Normal Gap):** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, **Diuretics/Diarrhea**, **RTA**, Pancreatic fistula). * **Key Distinction:** If the question mentions **Hyperchloremia**, always think of NAGMA (RTA or Diarrhea).

Question 159: A 40-year-old male presents with recurrent bouts of vomiting for 9 months because of pyloric obstruction. What is the compensatory biochemical change?

A. Respiratory Alkalosis
B. Respiratory acidosis
C. Paradoxical aciduria with hyponatremia and hypochloremia (Correct Answer)
D. Metabolic acidosis

Explanation: ### **Explanation** **Underlying Concept:** Pyloric obstruction leads to persistent vomiting, causing a massive loss of gastric hydrochloric acid (HCl). This results in **Metabolic Alkalosis** (due to loss of H⁺) and **Hypochloremia** (due to loss of Cl⁻). To compensate for the volume loss (dehydration), the body activates the Renin-Angiotensin-Aldosterone System (RAAS). **Why Option C is Correct:** 1. **Hyponatremia & Hypochloremia:** Vomiting directly removes Na⁺ and Cl⁻. 2. **The "Paradox":** Normally, in alkalosis, the kidneys should excrete bicarbonate (alkaline urine) to restore pH. However, in pyloric obstruction, the body prioritizes volume over pH. 3. **Mechanism:** To conserve Na⁺ (and water), the distal tubule reabsorbs Na⁺ in exchange for H⁺ (via the H⁺/Na⁺ exchanger) and K⁺. This occurs because Cl⁻ is unavailable to be reabsorbed with Na⁺. Consequently, H⁺ ions are secreted into the urine, making it **acidic** despite the systemic **alkalosis**. This is termed **Paradoxical Aciduria**. **Why Other Options are Incorrect:** * **A & B (Respiratory Changes):** While the lungs may attempt a minor compensatory hypoventilation (increasing CO₂), the primary biochemical hallmark and "paradox" of this condition is renal, not respiratory. * **D (Metabolic Acidosis):** Vomiting gastric contents causes a loss of acid, leading to alkalosis, not acidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Hypochloremic, hypokalemic, metabolic alkalosis with paradoxical aciduria. * **Key Electrolyte Shift:** Hypokalemia occurs because K⁺ is shifted into cells and excreted in the urine to save Na⁺. * **Treatment:** The definitive initial management is **Isotonic Saline (0.9% NaCl)**. The chloride in the saline allows the kidney to stop secreting H⁺, thus correcting the paradoxical aciduria.

Question 160: Hypercalcemia is seen in all of the following conditions except?

A. Sarcoidosis
B. Multiple myeloma
C. Chronic renal failure (Correct Answer)
D. Prolonged immobilization

Explanation: **Explanation:** The correct answer is **Chronic Renal Failure (CRF)** because it is typically associated with **hypocalcemia**, not hypercalcemia. In CRF, the kidneys fail to convert 25-hydroxyvitamin D into its active form, **1,25-dihydroxyvitamin D (Calcitriol)**, due to the loss of the 1-alpha-hydroxylase enzyme. Additionally, phosphate retention (hyperphosphatemia) leads to the precipitation of calcium, further lowering serum levels. This hypocalcemia triggers secondary hyperparathyroidism. (Note: Tertiary hyperparathyroidism in end-stage renal disease can cause hypercalcemia, but the classic presentation of CRF is low calcium). **Analysis of Incorrect Options:** * **Sarcoidosis:** Granulomatous diseases involve macrophages that possess 1-alpha-hydroxylase activity, leading to uncontrolled production of active Vitamin D and subsequent hypercalcemia. * **Multiple Myeloma:** Malignant plasma cells produce Osteoclast Activating Factors (OAFs) like IL-6 and RANK-L, causing extensive bone resorption and significant hypercalcemia. * **Prolonged Immobilization:** Lack of weight-bearing leads to an imbalance between bone formation and resorption. Increased osteoclastic activity releases calcium from the skeleton into the blood. **NEET-PG High-Yield Pearls:** * **Most common cause of hypercalcemia (Outpatient):** Primary Hyperparathyroidism. * **Most common cause of hypercalcemia (Inpatient):** Malignancy. * **Milk-Alkali Syndrome:** A triad of hypercalcemia, metabolic alkalosis, and renal failure due to excessive ingestion of calcium and absorbable antacids. * **ECG finding in Hypercalcemia:** Shortened QT interval ("Calcium shortens the heart's rest").

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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