Acid-Base and Electrolyte Balance — MCQs

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

Question 31: All of the following are causes of normal anion gap acidosis except?

A. Diarrhea
B. Hypoaldosteronism
C. Renal tubular acidosis
D. Aspirin overdose (Correct Answer)

Explanation: **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. **Why Aspirin Overdose is the correct answer:** Aspirin (Salicylate) poisoning causes a **High Anion Gap Metabolic Acidosis (HAGMA)**. Salicylates interfere with mitochondrial oxidative phosphorylation, leading to the accumulation of organic acids like lactate and ketoacids. These "unmeasured anions" increase the anion gap. Additionally, aspirin directly stimulates the respiratory center, often causing a primary respiratory alkalosis initially or as part of a mixed acid-base disorder. **Analysis of Incorrect Options (Causes of Normal Anion Gap/Hyperchloremic Acidosis):** In Normal Anion Gap Metabolic Acidosis (NAGMA), the loss of bicarbonate is compensated by a rise in chloride to maintain electroneutrality. * **Diarrhea:** The most common cause of NAGMA due to direct gastrointestinal loss of bicarbonate. * **Renal Tubular Acidosis (RTA):** Whether due to failure to reabsorb $HCO_3^-$ (Type 2) or failure to excrete $H^+$ (Type 1), the result is a renal loss of base with a normal AG. * **Hypoaldosteronism (Type 4 RTA):** Deficiency or resistance to aldosterone leads to hyperkalemia, which inhibits ammoniagenesis, resulting in decreased net acid excretion and NAGMA. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, **Lactate**, **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). * **Gold Standard:** In salicylate poisoning, the classic presentation is a **mixed** respiratory alkalosis and HAGMA.

Question 32: In a 40-year-old woman receiving total parenteral nutrition for a small-bowel fistula, what finding can be attributed to hypophosphatemia?

A. Increased cardiac output
B. Diarrhea
C. Increased energy production
D. Rhabdomyolysis (Correct Answer)

Explanation: **Explanation:** **1. Why Rhabdomyolysis is Correct:** Phosphorus is a critical component of **Adenosine Triphosphate (ATP)** and **2,3-Bisphosphoglycerate (2,3-BPG)**. In severe hypophosphatemia (often triggered by "Refeeding Syndrome" in TPN patients), intracellular ATP levels plummet. Since muscle cells require ATP to maintain membrane integrity and fuel the sodium-potassium pump, energy depletion leads to cell membrane dysfunction, leakage of intracellular contents, and eventual **rhabdomyolysis** (muscle breakdown). This can progress to acute renal failure due to myoglobinuria. **2. Why Other Options are Incorrect:** * **A. Increased cardiac output:** Hypophosphatemia actually causes **decreased** cardiac output. Low ATP impairs myocardial contractility, leading to congestive heart failure. * **B. Diarrhea:** Hypophosphatemia is associated with **ileus** (decreased bowel motility) rather than diarrhea, as smooth muscle contraction is impaired due to lack of energy. * **C. Increased energy production:** This is the opposite of the truth. Phosphorus is essential for the phosphorylation of glucose and the production of ATP; its deficiency causes a profound **energy crisis** at the cellular level. **3. NEET-PG High-Yield Pearls:** * **Refeeding Syndrome:** Classic cause of hypophosphatemia. Occurs when a malnourished patient (e.g., fistula, alcoholism, anorexia) is started on high-carbohydrate TPN. Insulin surge shifts phosphate into cells. * **Hematologic Effect:** Low 2,3-BPG shifts the oxygen-dissociation curve to the **left**, causing tissue hypoxia. It can also cause hemolysis due to rigid RBC membranes. * **Neuromuscular Effect:** Can lead to "metabolic encephalopathy" and respiratory failure due to diaphragmatic weakness.

Question 33: Which of the following conditions does not promote the dissociation of O2 from hemoglobin?

A. Decrease in pH
B. Increase in 2,3-diphosphoglycerate concentration
C. Increase in CO2 concentration
D. Increase in the partial pressure of O2 (Correct Answer)

Explanation: ### Explanation The dissociation of oxygen from hemoglobin is represented by the **Oxygen-Dissociation Curve (ODC)**. A shift to the **right** indicates decreased affinity (increased dissociation/unloading to tissues), while a shift to the **left** indicates increased affinity (decreased dissociation/loading in lungs). **Why Option D is Correct:** An **increase in the partial pressure of O2 ($PO_2$)** occurs in the pulmonary capillaries. According to the law of mass action and the cooperative binding property of hemoglobin, high $PO_2$ promotes the binding of oxygen to heme groups, stabilizing the **R (Relaxed) state**. This promotes the formation of oxyhemoglobin rather than dissociation. **Why the Other Options are Incorrect:** Options A, B, and C all cause a **Right Shift** in the ODC, promoting oxygen unloading (dissociation) via the stabilization of the **T (Tense) state**: * **Decrease in pH (Acidosis) & Increase in $CO_2$:** Known as the **Bohr Effect**. High $H^+$ and $CO_2$ (metabolic byproducts in tissues) reduce Hb's affinity for $O_2$. * **Increase in 2,3-DPG:** This glycolytic intermediate binds to the central cavity of the deoxyhemoglobin tetramer, stabilizing the T-state and pushing $O_2$ off the heme. --- ### High-Yield Clinical Pearls for NEET-PG * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis, **D**PG (2,3-DPG) increase, **E**xercise, **T**emperature increase. * **Fetal Hemoglobin (HbF):** Has a **Left Shift** compared to HbA because it has a lower affinity for 2,3-DPG, allowing it to "pull" oxygen from maternal blood. * **Carbon Monoxide (CO) Poisoning:** Causes a **Left Shift** of the remaining heme sites (increasing their affinity and preventing $O_2$ release to tissues) while simultaneously decreasing the total $O_2$ carrying capacity.

Question 34: Which of the following causes chloride-responsive metabolic alkalosis?

A. Recurrent vomiting (Correct Answer)
B. Bartter syndrome
C. Milk-alkali syndrome
D. Diuretic overdose

Explanation: **Explanation:** Metabolic alkalosis is clinically classified based on the **Urinary Chloride (UCl⁻)** concentration, which determines whether the condition will respond to saline (NaCl) infusion. **Correct Answer: A. Recurrent Vomiting** Vomiting leads to the loss of gastric HCl. This results in both hydrogen ion depletion and **hypochloremia**. The body attempts to maintain electroneutrality by retaining bicarbonate. Because the primary cause is a loss of chloride, providing intravenous saline (NaCl) restores chloride levels, allowing the kidneys to excrete the excess bicarbonate. Thus, it is **Chloride-Responsive (UCl⁻ < 10-20 mmol/L)**. **Incorrect Options:** * **B. Bartter Syndrome:** This is a genetic defect in the thick ascending limb of the Loop of Henle (mimicking loop diuretics). While it causes alkalosis, there is persistent urinary chloride wasting (**UCl⁻ > 20 mmol/L**), making it **Chloride-Resistant**. * **C. Milk-alkali Syndrome:** Caused by excessive intake of calcium and absorbable alkali. It is characterized by hypercalcemia and alkalosis but is not driven by chloride depletion; hence, it is **Chloride-Resistant**. * **D. Diuretic Overdose:** While *active* diuretic use causes chloride loss in urine, chronic states or syndromes mimicking them (like Bartter’s or Gitelman’s) are categorized as **Chloride-Resistant** because the kidney cannot retain chloride even if it is administered. **NEET-PG High-Yield Pearls:** 1. **Chloride-Responsive (UCl⁻ < 20):** Vomiting, Nasogastric suction, Laxative abuse, Diuretic use (remote). 2. **Chloride-Resistant (UCl⁻ > 20):** Mineralocorticoid excess (Conn’s, Cushing’s), Bartter/Gitelman syndrome, Severe hypokalemia. 3. **The "Saline Test":** If the alkalosis corrects with 0.9% NaCl, it is chloride-responsive.

Question 35: Which amino acid acts as a buffer at physiological pH of 7.4?

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

Explanation: **Explanation:** The buffering capacity of an amino acid depends on its **pKa value** (the pH at which the molecule is 50% ionized and 50% unionized). An amino acid acts as an effective buffer only when the ambient pH is close to its pKa (usually within ±1 pH unit). **Why Histidine is Correct:** Histidine is the only amino acid with an ionizable side chain (imidazole ring) that has a **pKa of approximately 6.0**. While 6.0 is slightly below the physiological pH of 7.4, it is close enough that Histidine remains the most effective buffer among all amino acids at this range. In proteins like **Hemoglobin**, the local environment of Histidine residues can shift their pKa closer to 7.0, making them crucial for maintaining blood pH (the Bohr effect). **Why the others are incorrect:** * **Glycine (Option A):** A simple amino acid with no ionizable side chain. Its pKa values are ~2.3 (carboxyl) and ~9.6 (amino), neither of which are near 7.4. * **Lysine (Option C) & Arginine (Option D):** These are basic amino acids. Their side chain pKa values are significantly higher (~10.5 for Lysine and ~12.5 for Arginine). At pH 7.4, they are almost entirely protonated and cannot act as buffers. **Clinical Pearls for NEET-PG:** * **Intracellular Buffering:** Histidine is the most important protein buffer in the intracellular fluid. * **Albumin & Hemoglobin:** These proteins have high buffering capacities primarily because they are rich in Histidine residues. * **Isoelectric Point (pI):** Remember that at pH 7.4, basic amino acids (His, Lys, Arg) carry a net positive charge, while acidic amino acids (Asp, Glu) carry a net negative charge.

Question 36: Hypermagnesemia may be observed in which of the following conditions?

A. Hyperparathyroidism
B. Diabetes mellitus (Correct Answer)
C. Kwashiorkor
D. Primary aldosteronism

Explanation: **Explanation:** **Correct Option: B. Diabetes Mellitus** Hypermagnesemia is primarily associated with **uncontrolled Diabetes Mellitus**, specifically during **Diabetic Ketoacidosis (DKA)**. While patients with chronic diabetes often have a total body magnesium deficit due to osmotic diuresis (glycosuria), they frequently present with *apparent* hypermagnesemia at the time of admission. This occurs due to: 1. **Insulin Deficiency:** Insulin normally promotes the intracellular shift of magnesium; its absence leads to magnesium moving out of cells. 2. **Hemoconcentration:** Severe dehydration and decreased renal perfusion (prerenal azotemia) reduce the renal clearance of magnesium, leading to elevated serum levels. **Analysis of Incorrect Options:** * **A. Hyperparathyroidism:** Parathyroid hormone (PTH) increases renal magnesium reabsorption. However, hypercalcemia (seen in hyperparathyroidism) actually promotes magnesium excretion (calciuresis-induced magnesiuria), often leading to **hypomagnesemia**. * **C. Kwashiorkor:** Protein-energy malnutrition is a classic cause of **hypomagnesemia** due to inadequate dietary intake and chronic diarrhea/malabsorption. * **D. Primary Aldosteronism:** Excess aldosterone causes ECF volume expansion, which inhibits proximal tubular reabsorption of magnesium, leading to increased urinary loss and **hypomagnesemia**. **NEET-PG High-Yield Pearls:** * **Most common cause of hypermagnesemia:** Renal failure (decreased excretion) or excessive intake (e.g., antacids, laxatives, or MgSO₄ therapy in eclampsia). * **Clinical Sign:** The earliest sign of hypermagnesemia is the **loss of deep tendon reflexes (DTRs)**, occurring at levels of 4–6 mEq/L. * **Antidote:** Intravenous **Calcium Gluconate** is the treatment of choice to antagonize the cardiotoxic effects of magnesium.

Question 37: High anion gap metabolic acidosis is seen in which of the following conditions?

A. Salicylate poisoning
B. Methanol poisoning
C. Starvation
D. All the above (Correct Answer)

Explanation: **Explanation:** Metabolic acidosis is characterized by a primary decrease in serum bicarbonate ($HCO_3^-$). It is classified based on the **Anion Gap (AG)**, calculated as: $AG = Na^+ - (Cl^- + HCO_3^-)$. The normal range is 8–12 mEq/L. A **High Anion Gap Metabolic Acidosis (HAGMA)** occurs when fixed acids (unmeasured anions) are added to the blood, consuming bicarbonate. **Analysis of Options:** * **Salicylate Poisoning:** Aspirin overdose causes HAGMA by interfering with oxidative phosphorylation, leading to the accumulation of lactic acid and ketoacids. (Note: It also uniquely causes a primary Respiratory Alkalosis). * **Methanol Poisoning:** Methanol is metabolized by alcohol dehydrogenase into **formic acid**, a potent unmeasured anion that significantly raises the anion gap. * **Starvation:** During prolonged fasting, the body shifts to fat metabolism, producing ketone bodies (acetoacetate and $\beta$-hydroxybutyrate). These organic acids dissociate, releasing $H^+$ ions and unmeasured anions, causing starvation ketoacidosis. Since all three conditions involve the addition of exogenous or endogenous acids, **Option D** is correct. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic for HAGMA (MUDPILES):** **M**ethanol, **U**remia, **D**KA, **P**araldehyde/Propylene glycol, **I**NH/Iron, **L**actic acidosis, **E**thylene glycol, **S**alicylates. 2. **Normal Anion Gap Metabolic Acidosis (NAGMA):** Also called hyperchloremic acidosis; common causes include Diarrhea and Renal Tubular Acidosis (RTA). 3. **Osmolar Gap:** If HAGMA is present with a high osmolar gap, suspect Methanol or Ethylene glycol poisoning.

Question 38: Which of the following is NOT true about serum calcium?

A. Approximately 25% of the serum calcium is in ionized form. (Correct Answer)
B. Total serum calcium levels typically range from 8.5 to 10.5 mg/dL.
C. Ionized calcium levels typically range from 4.4 to 5.2 mg/dL.
D. For each 1 g/dL alteration in serum albumin, there is a 0.8 mg/dL increase or decrease in protein-bound calcium.

Explanation: ### Explanation **1. Why Option A is the Correct Answer (The False Statement):** In healthy individuals, approximately **50%** of serum calcium exists in the **ionized (free) form**, which is the physiologically active fraction. The remaining calcium is distributed as protein-bound (about 40%, primarily to albumin) and complexed with anions like citrate or phosphate (about 10%). Therefore, stating that only 25% is ionized is incorrect. **2. Analysis of Other Options:** * **Option B:** The normal range for **total serum calcium** is indeed **8.5 to 10.5 mg/dL**. This represents the sum of all three fractions (ionized, protein-bound, and complexed). * **Option C:** The normal range for **ionized calcium** is approximately **4.4 to 5.2 mg/dL** (or 1.1 to 1.3 mmol/L). This is the fraction regulated by PTH and Vitamin D. * **Option D:** This describes the standard correction formula. Since 40% of calcium is bound to albumin, a change in albumin levels significantly affects *total* calcium but not necessarily the *ionized* fraction. The formula used is: *Corrected Calcium = Measured Total Calcium + [0.8 × (4.0 - Serum Albumin)]* **3. High-Yield Clinical Pearls for NEET-PG:** * **Acid-Base Influence:** Alkalosis increases the binding of calcium to albumin (decreasing ionized calcium), which can trigger **tetany** despite normal total calcium levels. Acidosis has the opposite effect. * **Active Form:** Only the **ionized fraction** is biologically active and exerts feedback on the parathyroid glands. * **Sample Collection:** For ionized calcium, samples should be collected anaerobically to prevent CO2 loss (which would increase pH and falsely lower ionized calcium levels). * **Hypocalcemia Sign:** Look for **Chvostek's sign** (facial twitching) and **Trousseau's sign** (carpal spasm) in clinical vignettes.

Question 39: What is the normal plasma osmolality?

A. 285-300 milliosmo/kg (Correct Answer)
B. 310-340 milliosmo/kg
C. 200-240 milliosmo/kg
D. 160-180 milliosmo/kg

Explanation: **Explanation:** **Plasma osmolality** is a measure of the concentration of solutes (particles) per kilogram of solvent (water) in the blood. The correct range is **285–300 mOsm/kg**. This tight physiological range is maintained primarily by the hypothalamus-pituitary-renal axis through the action of Antidiuretic Hormone (ADH) and the thirst mechanism. The primary determinants of plasma osmolality are **Sodium ($Na^+$)**, **Glucose**, and **Blood Urea Nitrogen (BUN)**. Sodium is the most significant contributor because it is the most abundant extracellular cation. **Analysis of Options:** * **Option A (Correct):** 285–300 mOsm/kg is the standard physiological reference range. Some textbooks may cite 275–295 mOsm/kg, but 285–300 is the most commonly accepted range in clinical biochemistry exams. * **Option B:** 310–340 mOsm/kg represents **Hyperosmolality**. This occurs in severe dehydration, Diabetes Insipidus, or Hyperglycemic Hyperosmolar State (HHS). * **Options C & D:** 160–240 mOsm/kg represents severe **Hypoosmolality**, which is clinically seen in conditions like SIADH (Syndrome of Inappropriate ADH) or water intoxication, leading to cerebral edema. **High-Yield Clinical Pearls for NEET-PG:** 1. **Calculated Osmolality Formula:** $2 \times [Na^+] + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8}$. 2. **Osmolar Gap:** The difference between measured (laboratory) and calculated osmolality. A gap **>10 mOsm/kg** suggests the presence of unmeasured toxins like Ethanol, Methanol, or Ethylene glycol. 3. **Major Osmolyte:** Sodium and its associated anions ($Cl^-$ and $HCO_3^-$) account for nearly 90% of plasma osmolality.

Question 40: Hypophosphatemia is seen in all of the following conditions EXCEPT?

A. Acute renal failure (Correct Answer)
B. Resolving phase of diabetic ketoacidosis
C. Respiratory alkalosis / COPD
D. Chronic alcoholism

Explanation: **Explanation:** **1. Why Acute Renal Failure (ARF) is the Correct Answer:** In **Acute Renal Failure**, there is a sudden decline in the Glomerular Filtration Rate (GFR). This leads to the retention of phosphate because the kidneys are unable to excrete it. Consequently, ARF typically presents with **Hyperphosphatemia**, not hypophosphatemia. This is often accompanied by hypocalcemia due to the reciprocal relationship between calcium and phosphate. **2. Why the other options are incorrect (Causes of Hypophosphatemia):** * **Resolving phase of DKA:** During treatment with insulin, phosphate (along with potassium and glucose) shifts from the extracellular fluid into the cells. Furthermore, the osmotic diuresis during the acidotic phase leads to total body phosphate depletion. * **Respiratory Alkalosis:** This is a classic cause of "intracellular shift." Alkalosis activates phosphofructokinase, stimulating glycolysis, which consumes inorganic phosphate to produce phosphorylated glycolytic intermediates, leading to a rapid drop in serum phosphate levels. * **Chronic Alcoholism:** This is one of the most common causes of severe hypophosphatemia due to a combination of poor dietary intake, vitamin D deficiency, and ethanol-induced tubular dysfunction leading to phosphaturia. **NEET-PG High-Yield Pearls:** * **Refeeding Syndrome:** A high-yield clinical scenario where a malnourished patient (or alcoholic) is given carbohydrates, leading to insulin release and profound hypophosphatemia, which can cause cardiac failure. * **Fanconi Syndrome:** A proximal tubular defect that leads to hypophosphatemia due to decreased renal reabsorption (phosphaturia). * **Antacids:** Chronic use of aluminum or magnesium hydroxide antacids can cause hypophosphatemia by binding phosphate in the gut.

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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