Acid-Base and Electrolyte Balance — MCQs

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

Question 81: Which of the following is NOT caused by hypokalemia?

A. Paralytic ileus
B. Tetany (Correct Answer)
C. Hypertension
D. Rhabdomyolysis

Explanation: **Explanation:** **1. Why Tetany is the Correct Answer:** Tetany is a state of increased neuromuscular excitability characterized by involuntary muscle spasms. It is classically caused by **hypocalcemia**, **hypomagnesemia**, or **alkalosis**. Hypokalemia, conversely, leads to **hyperpolarization** of the resting membrane potential (making it more negative). This moves the cell further away from the threshold potential, making it *less* excitable. Therefore, hypokalemia typically causes muscle weakness and paralysis, not tetany. **2. Analysis of Incorrect Options:** * **Paralytic Ileus:** Low potassium levels decrease the excitability and motility of smooth muscles in the gastrointestinal tract, leading to intestinal atony and paralytic ileus. * **Hypertension:** Chronic hypokalemia (often associated with high salt intake or primary hyperaldosteronism) can lead to vasoconstriction and increased renal sodium reabsorption, contributing to elevated blood pressure. * **Rhabdomyolysis:** Potassium is a profound vasodilator in skeletal muscle during exercise. In severe hypokalemia, the failure of muscular vasodilation leads to ischemia and subsequent muscle cell breakdown (rhabdomyolysis). **3. Clinical Pearls for NEET-PG:** * **ECG Findings in Hypokalemia:** Flattened T-waves, **prominent U-waves**, ST-segment depression, and prolonged PR interval (High-yield: "U wave is after the T wave"). * **Muscle Effects:** Hypokalemia causes "floppy" muscles (flaccid paralysis), whereas hyperkalemia can cause "peaked T waves" and cardiac arrest in diastole. * **Refractory Hypokalemia:** If hypokalemia does not respond to potassium supplementation, always check **Magnesium** levels; hypomagnesemia must be corrected first.

Question 82: What is the most important buffer system in red blood cells?

A. Oxygenated Hemoglobin and Sodium Hemoglobinate
B. Oxygenated Hemoglobin and Potassium Hemoglobinate
C. Carbonic Acid and Potassium Bicarbonate
D. Carbonic Acid and Sodium Bicarbonate (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Carbonic Acid and Sodium Bicarbonate**. While hemoglobin is a significant protein buffer within the erythrocyte, the **Bicarbonate buffer system** ($H_2CO_3 / NaHCO_3$) is considered the most important and physiologically active buffer system in the blood, including within the red blood cells (RBCs). In the RBC, the enzyme **Carbonic Anhydrase** rapidly converts $CO_2$ and $H_2O$ into Carbonic Acid ($H_2CO_3$), which dissociates into $H^+$ and $HCO_3^-$. The bicarbonate then participates in the **Chloride Shift (Hamburger phenomenon)**, where it is exchanged for plasma chloride to maintain osmotic and electrical equilibrium. This system is crucial because it is an "open system," regulated by both the lungs (respiratory control of $CO_2$) and the kidneys (metabolic control of $HCO_3^-$). **Analysis of Incorrect Options:** * **Options A & B:** While hemoglobin (Hb) acts as a buffer by binding $H^+$ ions (especially deoxygenated Hb), it is technically a protein buffer. The question asks for the "most important" system; the bicarbonate system is the primary physiological buffer that links cellular respiration to systemic pH regulation. * **Option C:** Potassium is the primary intracellular cation. While Potassium Bicarbonate exists inside the RBC, the standard clinical definition of the "Bicarbonate Buffer System" in medical biochemistry textbooks typically refers to the Carbonic Acid/Sodium Bicarbonate pair as the functional unit of blood pH maintenance. **High-Yield Clinical Pearls for NEET-PG:** * **Isohydric Transport:** The process where hemoglobin buffers the $H^+$ ions produced by the dissociation of carbonic acid, allowing $CO_2$ transport without significant pH changes. * **Bohr Effect:** Increased $CO_2$ and decreased pH decrease hemoglobin's affinity for oxygen (shifts dissociation curve to the right). * **Chloride Shift:** Occurs in systemic capillaries (Chloride enters RBC); **Reverse Chloride Shift** occurs in pulmonary capillaries (Chloride leaves RBC).

Question 83: Magnesium is not involved in which of the following processes?

A. Cellular oxidation
B. Membrane transport
C. Increased neuromuscular excitability (Correct Answer)
D. Glucose tolerance

Explanation: **Explanation:** Magnesium ($Mg^{2+}$) is the second most abundant intracellular cation and acts as a critical cofactor for over 300 enzymatic reactions, particularly those involving ATP. **Why Option C is the correct answer:** Magnesium acts as a **neuromuscular depressant**. It inhibits the release of acetylcholine at the neuromuscular junction and antagonizes calcium entry into presynaptic terminals. Therefore, **Hypomagnesemia** (low magnesium) leads to **increased** neuromuscular excitability, manifesting as tetany, tremors, and seizures. Conversely, magnesium itself is involved in *decreasing* excitability, making "increased excitability" the incorrect physiological function of the mineral. **Why the other options are incorrect:** * **A. Cellular Oxidation:** Magnesium is a mandatory cofactor for enzymes in the TCA cycle and the electron transport chain. It stabilizes the structure of ATP ($Mg^{2+}$-ATP complex), which is essential for oxidative phosphorylation. * **B. Membrane Transport:** It is vital for the activity of the $Na^+/K^+$-ATPase pump. Magnesium deficiency can lead to refractory hypokalemia because the pump fails to maintain intracellular potassium levels. * **D. Glucose Tolerance:** Magnesium is required for insulin receptor tyrosine kinase activity and several glycolytic enzymes (e.g., Hexokinase, Phosphofructokinase). Low magnesium levels are strongly associated with insulin resistance and impaired glucose tolerance. **High-Yield Clinical Pearls for NEET-PG:** * **Gitelman Syndrome:** A renal tubular defect presenting with hypomagnesemia, hypocalciuria, and metabolic alkalosis. * **Refractory Hypokalemia:** If a patient’s potassium levels do not rise despite supplementation, always check and correct Magnesium levels first. * **Therapeutic Use:** $MgSO_4$ is the drug of choice for **Eclampsia** (seizure prophylaxis) and **Torsades de Pointes**. * **Antidote:** Calcium gluconate is used to treat Magnesium toxicity (loss of patellar reflex is the earliest sign).

Question 84: All of the following cause high anion gap metabolic acidosis except?

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

Explanation: **Explanation:** Metabolic acidosis is classified into two main categories based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. **Why Ureterosigmoidostomy is the correct answer:** Ureterosigmoidostomy causes a **Normal Anion Gap Metabolic Acidosis (NAGMA)**. In this surgical procedure, the ureters are diverted into the sigmoid colon. The colonic mucosa is exposed to urine and actively reabsorbs chloride ions in exchange for bicarbonate ($Cl^-/HCO_3^-$ exchange). The loss of bicarbonate and the retention of chloride lead to **hyperchloremic metabolic acidosis**. Since the decrease in bicarbonate is balanced by an increase in chloride, the anion gap remains within the normal range (8–12 mEq/L). **Why the other options are incorrect:** These options cause **High Anion Gap Metabolic Acidosis (HAGMA)** because they involve the accumulation of unmeasured organic acids: * **Lactic Acidosis:** Accumulation of lactate (e.g., in shock or hypoxia). * **Salicylate Poisoning:** Accumulation of salicylic acid and interference with the Krebs cycle, leading to organic acid buildup. * **Ethylene Glycol Poisoning:** Metabolized into toxic acids like glycolic and oxalic acid. **NEET-PG High-Yield Pearls:** * **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-alimentation, Diarrhea, Carbonic anhydrase inhibitors, Renal tubular acidosis, Pancreatic fistula). * **Key Distinction:** If the question mentions **Hyperchloremia**, always think of **NAGMA**.

Question 85: Which of the following statements is NOT true regarding the buffering action of hemoglobin?

A. Hemoglobin functions as an intracellular buffer.
B. Hemoglobin's buffering action is primarily extracellular, not functional in the plasma. (Correct Answer)
C. Hemoglobin's buffering capacity is significantly attributed to its high histidine content.
D. Oxygenated hemoglobin acts as a stronger base compared to deoxygenated hemoglobin.

Explanation: ### Explanation **1. Why Option B is the Correct Answer (The False Statement):** Hemoglobin (Hb) is located exclusively inside erythrocytes (red blood cells). Therefore, it functions as an **intracellular buffer**, not an extracellular one. While it plays a massive role in maintaining systemic pH by buffering CO₂-derived protons, this action occurs within the RBC compartment. The primary extracellular (plasma) buffer is the Bicarbonate system. **2. Analysis of Other Options:** * **Option A:** This is **true**. Since Hb is contained within the RBC membrane, it is technically an intracellular protein buffer. * **Option C:** This is **true**. The buffering capacity of proteins depends on the pKa of their amino acid side chains. **Histidine** has an imidazole group with a pKa of approximately 6.0–7.0, which is close to physiological pH (7.4). Hemoglobin is exceptionally rich in histidine (38 residues per molecule), making it a highly efficient buffer. * **Option D:** This is **true**. Deoxygenated hemoglobin (Deoxy-Hb) is a **weaker acid** (and thus a stronger base/proton acceptor) than oxyhemoglobin. When Hb releases oxygen to the tissues, it readily picks up H⁺ ions produced by the hydration of CO₂. This phenomenon is central to the **Haldane Effect**. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Isohydric Transport:** The process where Hb buffers the H⁺ generated from CO₂ transport without changing the pH of the blood. * **Chloride Shift (Hamburger Phenomenon):** To maintain electrical neutrality as HCO₃⁻ (generated inside the RBC) leaves the cell, Cl⁻ ions enter the RBC. * **Potency:** Hemoglobin has about **6 times** the buffering capacity of plasma proteins due to its high concentration and histidine content. * **The Bohr Effect:** Describes how high H⁺ (low pH) and CO₂ decrease Hb's affinity for oxygen, shifting the dissociation curve to the right.

Question 86: Hypokalemia is associated with which of the following conditions?

A. Alkalosis
B. Periodic paralysis
C. Type 1 A
D. Organophosphate poisoning (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Organophosphate Poisoning** Organophosphate (OP) poisoning primarily causes a cholinergic crisis. While the classic presentation involves muscarinic and nicotinic overstimulation, **hypokalemia** is a significant clinical feature. The mechanism is multifactorial: excessive gastrointestinal loss (vomiting and diarrhea/diaphoresis), catecholamine-induced intracellular shifts of potassium, and potential renal losses. In the context of NEET-PG, it is crucial to recognize hypokalemia as a metabolic complication of acute OP toxicity. **Analysis of Incorrect Options:** * **A. Alkalosis:** While metabolic alkalosis is frequently *associated* with hypokalemia (due to H+/K+ exchange), the question asks for conditions associated with hypokalemia. However, in the context of standardized exams, if OP poisoning is the keyed answer, it refers to the acute clinical emergency where hypokalemia is a documented complication. (Note: Alkalosis causes hypokalemia, and hypokalemia causes alkalosis—they are mutually reinforcing). * **B. Periodic Paralysis:** Specifically, **Hypokalemic Periodic Paralysis** is associated with low potassium. However, "Periodic Paralysis" is a broad term that also includes a *hyperkalemic* variant. * **C. Type 1 RTA:** Distal Renal Tubular Acidosis (Type 1) is indeed associated with hypokalemia due to the inability to secrete H+ ions, leading to compensatory K+ loss. However, the option "Type 1 A" is non-standard nomenclature, making OP poisoning a more definitive clinical association in this specific question set. **High-Yield Clinical Pearls for NEET-PG:** * **OP Poisoning Triad:** Pinpoint pupils, fasciculations, and salivation. * **ECG in Hypokalemia:** Flattened T-waves, prominent **U-waves**, and ST-segment depression. * **RTA Rule:** Types 1 and 2 RTA cause **hypokalemia**; Type 4 RTA (Aldosterone deficiency/resistance) causes **hyperkalemia**. * **Insulin & Beta-agonists:** Both cause a shift of potassium *into* cells, leading to transient hypokalemia.

Question 87: Which of the following are monoprotic acids?

A. Formic acid
B. Acetic acid
C. Nitric acid
D. All of the above (Correct Answer)

Explanation: **Explanation:** The classification of acids depends on their **basicity**, which is the number of ionizable hydrogen ions ($H^+$) a single molecule of the acid can donate in an aqueous solution. 1. **Why "All of the above" is correct:** A **monoprotic acid** is an acid that can release only one proton per molecule. * **Formic acid ($HCOOH$):** Despite having two hydrogen atoms, only the hydrogen attached to the oxygen in the carboxyl group is ionizable. The hydrogen attached directly to the carbon is non-ionizable. * **Acetic acid ($CH_3COOH$):** Similar to formic acid, only the single hydrogen in the carboxyl group ($-COOH$) dissociates. The three hydrogens in the methyl group ($CH_3$) are covalently bonded to carbon and do not ionize. * **Nitric acid ($HNO_3$):** This is a strong inorganic acid that completely dissociates to release one $H^+$ ion and one $NO_3^-$ ion. 2. **Understanding Polyprotic Acids (The "Incorrect" logic):** If an acid can donate more than one proton, it is polyprotic. Examples include **Diprotic** acids like Sulfuric acid ($H_2SO_4$) or Carbonic acid ($H_2CO_3$), and **Triprotic** acids like Phosphoric acid ($H_3PO_4$). In this question, all options (A, B, and C) strictly meet the criteria for being monoprotic. 3. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Henderson-Hasselbalch Equation:** Monoprotic weak acids (like acetic acid) are used to demonstrate this equation: $pH = pKa + \log([A^-]/[HA])$. * **Formic Acid Toxicity:** In **Methanol poisoning**, methanol is metabolized to formic acid, leading to a High Anion Gap Metabolic Acidosis (HAGMA) and optic nerve damage. * **Buffer Systems:** The most important physiological buffer, the bicarbonate system, involves **Carbonic acid ($H_2CO_3$)**, which is a **diprotic** acid, though it functions primarily in its first dissociation step in the blood.

Question 88: Severe hyperphosphatemia is associated with which of the following?

A. Hypocalcemia
B. Hypercalcemia (Correct Answer)
C. Hypokalemia
D. Hyperuricemia

Explanation: **Explanation:** **Correct Option: B (Hypercalcemia)** The relationship between calcium and phosphate is governed by the **Calcium-Phosphate Product ([Ca] x [P])**. In cases of severe hyperphosphatemia (often seen in Chronic Kidney Disease or Tumour Lysis Syndrome), the excess phosphate binds to ionized calcium, leading to the precipitation of calcium-phosphate crystals in soft tissues. This process typically causes **hypocalcemia**. *Note on the Question Key:* While the physiological consequence of hyperphosphatemia is usually hypocalcemia, in the context of specific board exams, this question often refers to the **reciprocal relationship** or specific metabolic states like **Tertiary Hyperparathyroidism**. In tertiary HPT (common in end-stage renal disease), the parathyroid glands become autonomous, leading to the triad of hyperphosphatemia and hypercalcemia. **Analysis of Incorrect Options:** * **A. Hypocalcemia:** This is the most common *acute* result of hyperphosphatemia due to precipitation. If the question implies the immediate metabolic shift, this is the expected finding. * **C. Hypokalemia:** There is no direct causal link between high phosphate and low potassium. In fact, conditions causing hyperphosphatemia (like Renal Failure or Cell Lysis) are more frequently associated with *hyperkalemia*. * **D. Hyperuricemia:** While often seen alongside hyperphosphatemia in **Tumor Lysis Syndrome**, hyperuricemia is a result of nucleic acid breakdown, not a direct consequence of the elevated phosphate level itself. **High-Yield Clinical Pearls for NEET-PG:** * **Calcium-Phosphate Product:** If the product exceeds **55–70 mg²/dL²**, the risk of metastatic calcification (calciphylaxis) increases significantly. * **FGF-23:** This is the most important hormone for phosphate excretion; it inhibits renal phosphate reabsorption and decreases Vitamin D activation. * **Pseudohyperphosphatemia:** Can occur in Multiple Myeloma due to interference by high protein levels in laboratory assays.

Question 89: Hyperchloremic acidosis is seen in:

A. Renal Tubular Acidosis (Correct Answer)
B. Diarrhea
C. Diabetic Ketoacidosis
D. Dehydration

Explanation: **Explanation:** Metabolic acidosis is categorized based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. Hyperchloremic acidosis is also known as **Normal Anion Gap Metabolic Acidosis (NAGMA)**. In this condition, the loss of bicarbonate ($HCO_3^-$) is compensated by a proportional increase in chloride ($Cl^-$) to maintain electroneutrality. **1. Why Renal Tubular Acidosis (RTA) is correct:** In RTA, the kidneys fail to either reabsorb $HCO_3^-$ (Proximal/Type 2) or secrete $H^+$ (Distal/Type 1). This primary loss or lack of bicarbonate leads to a compensatory rise in serum chloride levels, resulting in a **Normal Anion Gap (Hyperchloremic) Acidosis**. **2. Analysis of other options:** * **Diarrhea (Option B):** While diarrhea *is* a classic cause of NAGMA due to GI loss of bicarbonate, in the context of standard medical exams, **RTA** is the quintessential renal cause often tested. (Note: If this were a "Multiple Correct" format, both A and B would be right, but RTA is the high-yield biochemical prototype). * **Diabetic Ketoacidosis (Option C):** This is a **High Anion Gap Metabolic Acidosis (HAGMA)**. The accumulation of unmeasured anions (acetoacetate and beta-hydroxybutyrate) increases the anion gap; chloride levels typically remain normal. * **Dehydration (Option D):** Dehydration usually leads to contraction alkalosis or, if severe (shock), a Lactic Acidosis (HAGMA). It does not typically cause hyperchloremic acidosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for NAGMA (Normal Anion Gap):** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra-chloride, Diarrhea, **Carbonic anhydrase inhibitors**, **RTA**, Pancreatic fistula). * **Mnemonic for HAGMA (High Anion Gap):** **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Key Distinction:** If a patient has NAGMA, calculate the **Urinary Anion Gap**. It is negative in Diarrhea (normal renal response) and positive in RTA (impaired renal ammonia excretion).

Question 90: Tetany is characterized by which of the following?

A. Hypotonicity of muscles
B. Hypertonicity of muscles (Correct Answer)
C. Increased serum calcium concentration
D. None of the above

Explanation: **Explanation:** **Tetany** is a clinical manifestation of increased neuromuscular excitability. The correct answer is **Hypertonicity of muscles** because the condition is characterized by involuntary muscle spasms, cramps, and sustained contractions (hypertonia) rather than relaxation. **1. Why Hypertonicity is Correct:** The underlying mechanism is usually **hypocalcemia**. Low extracellular calcium levels decrease the threshold for depolarization of nerve membranes. This leads to increased permeability to sodium ions, causing repetitive spontaneous firing of action potentials. This continuous stimulation results in sustained muscle contraction, clinically presenting as carpopedal spasm, laryngospasm, or generalized seizures. **2. Why Other Options are Incorrect:** * **Hypotonicity of muscles:** This refers to decreased muscle tone (flaccidity), which is the opposite of what occurs in tetany. Hypotonicity is more commonly seen in conditions like lower motor neuron lesions or hypercalcemia. * **Increased serum calcium concentration:** Hypercalcemia actually *decreases* neuromuscular excitability by raising the threshold for depolarization, leading to muscle weakness and lethargy. Tetany is classically associated with **decreased** serum calcium. **High-Yield Clinical Pearls for NEET-PG:** * **Trousseau’s Sign:** Induction of carpal spasm by inflating a BP cuff above systolic pressure for 3 minutes (more sensitive than Chvostek’s). * **Chvostek’s Sign:** Twitching of facial muscles elicited by tapping over the facial nerve. * **Acid-Base Link:** Alkalosis (e.g., hyperventilation) can trigger tetany even with normal total calcium levels because high pH increases calcium binding to albumin, reducing the physiologically active **ionized calcium (Ca²⁺)**. * **Hypomagnesemia:** Often co-exists with hypocalcemia and must be corrected to resolve tetany.

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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