Acid-Base and Electrolyte Balance — MCQs

A junior doctor had difficulty in determining the base deficit/excess for blood in a given patient. An experienced senior resident advised a quick method to determine the acid-base composition of blood based on pCO2. Which of the following is the likely method he suggested to predict acid-base composition of blood?

Q18Easy

What is the equivalent weight of Ca2+ ion?

Q19Easy

Increased anion gap acidosis is seen in which of the following conditions?

Q20Easy

Hypomagnesemia is associated with which of the following electrolyte imbalances?

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

Question 11: Thyroxine is synthesized from which amino acid?

A. Phenylalanine
B. Tryptophan
C. Tyrosine (Correct Answer)
D. None of the above

Explanation: **Explanation:** Thyroxine ($T_4$) and Triiodothyronine ($T_3$) are iodine-containing hormones synthesized in the follicular cells of the thyroid gland. The correct answer is **Tyrosine** because it serves as the structural backbone for these hormones. 1. **Why Tyrosine is Correct:** The synthesis begins with the protein **Thyroglobulin**, which contains multiple Tyrosine residues. Through the action of the enzyme *Thyroid Peroxidase (TPO)*, iodine is attached to these residues to form Monoiodotyrosine (MIT) and Diiodotyrosine (DIT). The coupling of two DIT molecules forms $T_4$, while the coupling of one MIT and one DIT forms $T_3$. 2. **Why Other Options are Incorrect:** * **Phenylalanine:** While Phenylalanine is the precursor to Tyrosine (via phenylalanine hydroxylase), it does not directly undergo iodination or coupling to form thyroid hormones. * **Tryptophan:** This is the precursor for Serotonin, Melatonin, and Niacin (Vitamin $B_3$), but it plays no role in thyroid hormone synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor Rule:** Tyrosine is also the precursor for **Catecholamines** (Dopamine, Norepinephrine, Epinephrine) and **Melanin**. * **Rate-limiting step:** The "Organification" of iodine (binding to Tyrosine) is a critical step inhibited by antithyroid drugs like Propylthiouracil and Methimazole. * **Wolff-Chaikoff Effect:** An autoregulatory phenomenon where high levels of circulating iodide temporarily inhibit the organification of iodine, reducing thyroid hormone synthesis.

Question 12: Which of the following is NOT a feature of hypocalcemia?

A. Numbness and tingling
B. Circumoral paresthesia
C. Depressed tendon reflexes (Correct Answer)
D. Skin irritability and sensitivity

Explanation: **Explanation:** The correct answer is **C. Depressed tendon reflexes**. In hypocalcemia, the hallmark finding is **hyperreflexia** (increased tendon reflexes), not depression. **1. Why Depressed Tendon Reflexes is the correct (incorrect feature) answer:** Calcium ions normally stabilize the resting membrane potential of excitable tissues. Low extracellular calcium levels lower the threshold for depolarization, making nerve and muscle cells **hyperexcitable**. This leads to spontaneous firing of motor neurons, resulting in hyperreflexia, tetany, and muscle cramps. Conversely, **depressed** or absent reflexes are characteristic of **hypercalcemia** or hypermagnesemia, where the membrane is stabilized and harder to depolarize. **2. Analysis of Incorrect Options (Features of Hypocalcemia):** * **A & B (Numbness, tingling, and circumoral paresthesia):** These are the earliest sensory symptoms of hypocalcemia. Increased excitability of sensory nerve fibers leads to "pins and needles" sensations, particularly around the mouth (circumoral) and in the fingertips. * **D (Skin irritability and sensitivity):** Chronic hypocalcemia can lead to dermatological manifestations, including dry, scaly skin, eczema, and increased sensitivity/irritability of the skin. **High-Yield Clinical Pearls for NEET-PG:** * **Chvostek’s Sign:** Tapping the facial nerve leads to twitching of facial muscles. * **Trousseau’s Sign:** Carpopedal spasm induced by inflating a BP cuff above systolic pressure (more specific than Chvostek’s). * **ECG Finding:** The classic sign is **prolonged QT interval** (due to lengthening of the ST segment). * **Mnemonic for Hypocalcemia:** "CATS go Numb" – **C**onvulsions, **A**rrhythmias, **T**etany, **S**pasms, and **Numbness**.

Question 13: Anion gap decreases with which of the following conditions?

A. Plasma cell dyscrasia
B. Ethylene glycol toxicity (Correct Answer)
C. Protein losing enteropathy
D. Nephrotic syndrome

Explanation: **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. It represents unmeasured anions in the plasma. **Why Ethylene Glycol is Correct:** Ethylene glycol toxicity is a classic cause of a **High Anion Gap Metabolic Acidosis (HAGMA)**. When metabolized by alcohol dehydrogenase, it produces glycolic and oxalic acids. these organic acids release protons (consumed by bicarbonate) and leave behind unmeasured anions (glycolate/oxalate), thereby **increasing** the anion gap. *Note: There appears to be a discrepancy in the question stem provided; Ethylene glycol **increases** the AG. However, based on the options provided, if the question asks for a decrease, the answer should typically be Hypoalbuminemia or Multiple Myeloma.* **Analysis of Other Options (Causes of Decreased Anion Gap):** 1. **Plasma Cell Dyscrasia (Multiple Myeloma):** This is a classic cause of a **decreased** anion gap. Myeloma proteins (IgG) are often cationic (positively charged) at physiological pH. To maintain electroneutrality, the body retains chloride (measured anion), which narrows the gap between sodium and measured anions. 2. **Protein-Losing Enteropathy & Nephrotic Syndrome:** Both conditions lead to **Hypoalbuminemia**. Albumin is the major unmeasured anion in plasma. A decrease in albumin reduces the total negative charge in the "gap," leading to a **decreased** anion gap. (Rule: For every 1 g/dL drop in albumin, the AG decreases by ~2.5 mEq/L). **High-Yield Clinical Pearls for NEET-PG:** * **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates) all **increase** the AG. * **Decreased AG** is most commonly caused by **Hypoalbuminemia**, Hypercalcemia, Hypermagnesemia, or Lithium toxicity. * **Normal Anion Gap Metabolic Acidosis (NAGMA)** is seen in RTA and Diarrhea (due to compensatory increase in Chloride).

Question 14: What is the earliest symptom of Tay-Sachs disease?

A. Exaggerated startle response (Correct Answer)
B. Bone deformation
C. Hepatomegaly
D. Excessive bleeding

Explanation: ### Explanation **Tay-Sachs Disease (TSD)** is an autosomal recessive lysosomal storage disorder caused by a deficiency of the enzyme **Hexosaminidase A**. This leads to the toxic accumulation of **GM2 gangliosides** within the lysosomes of neurons, resulting in progressive neurodegeneration. **Why "Exaggerated Startle Response" is correct:** The earliest clinical sign of Tay-Sachs disease, typically appearing between **3 to 6 months of age**, is an exaggerated startle response (hyperacusis) to loud noises. This occurs due to early neuronal irritability and dysfunction in the central nervous system before gross motor regression or blindness (amaurosis) becomes apparent. **Why the other options are incorrect:** * **B. Bone deformation:** This is characteristic of **Gaucher disease** (Erlenmeyer flask deformity) or **Hurler syndrome** (dysostosis multiplex), but not Tay-Sachs. * **C. Hepatomegaly:** A crucial diagnostic differentiator is that Tay-Sachs has **no hepatosplenomegaly**. Its presence would instead suggest **Niemann-Pick disease** (Sphingomyelinase deficiency) or Gaucher disease. * **D. Excessive bleeding:** This is not a feature of sphingolipidoses; it is typically associated with platelet disorders or coagulation factor deficiencies. **High-Yield Clinical Pearls for NEET-PG:** * **Cherry-red spot on macula:** A classic finding in both Tay-Sachs and Niemann-Pick. * **Enzyme Deficiency:** Hexosaminidase **A** (Tay-S**A**chs). * **Accumulated Substance:** GM2 Ganglioside. * **Histology:** "Onion-skin" appearance of lysosomes. * **Key Differentiator:** Tay-Sachs = No Hepatosplenomegaly; Niemann-Pick = Hepatosplenomegaly present.

Question 15: What is the chemical process involved in the conversion of progesterone to glucocorticoids?

A. Methylation
B. Hydroxylation (Correct Answer)
C. Carboxylation
D. None of the above

Explanation: **Explanation:** The conversion of progesterone to glucocorticoids (such as cortisol) is a fundamental pathway in steroidogenesis occurring in the adrenal cortex. This process is driven by a series of **Hydroxylation** reactions. **1. Why Hydroxylation is Correct:** Steroid hormones are derived from cholesterol (a 27-carbon molecule). Progesterone (a 21-carbon molecule) serves as a precursor for cortisol. To convert progesterone into cortisol, specific hydroxyl groups (-OH) must be added at specific carbon positions. This is mediated by **Cytochrome P450 enzymes**: * **21-hydroxylase:** Converts progesterone to 11-deoxycorticosterone. * **17α-hydroxylase:** Converts progesterone to 17-hydroxyprogesterone. * **11β-hydroxylase:** Final step in cortisol synthesis. Since the primary chemical change is the addition of -OH groups, the process is hydroxylation. **2. Why Other Options are Incorrect:** * **Methylation:** Involves adding a methyl group (-CH3). This is common in DNA regulation or catecholamine metabolism (e.g., PNMT enzyme), but not in the conversion of progesterone to glucocorticoids. * **Carboxylation:** Involves adding a carboxyl group (-COOH), typically requiring Biotin (Vitamin B7). Examples include pyruvate carboxylase or acetyl-CoA carboxylase. **Clinical Pearls for NEET-PG:** * **Congenital Adrenal Hyperplasia (CAH):** Most commonly caused by a deficiency in **21-hydroxylase** (90% of cases), leading to decreased cortisol and increased androgens. * **Rate-limiting step of steroidogenesis:** The conversion of cholesterol to pregnenolone by the enzyme **Desmolase** (Cholesterol side-chain cleavage enzyme). * **Localization:** Hydroxylation reactions in steroid synthesis occur in both the **Mitochondria** and the **Smooth Endoplasmic Reticulum**.

Question 16: Carbonic anhydrase is an enzyme that occurs in plants, bacteria, and animals and is involved in the formation of which chemical?

A. Carbon dioxide from carbon and oxygen
B. Carbonic acid from carbon dioxide and water (Correct Answer)
C. Bicarbonate ion from carbonic acid
D. Hydrochloric acid

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Carbonic anhydrase (CA) is a zinc-containing metalloenzyme that catalyzes the reversible hydration of carbon dioxide ($CO_2$). The primary reaction involves the combination of $CO_2$ and water ($H_2O$) to form **carbonic acid ($H_2CO_3$)**. This reaction is essential for the transport of $CO_2$ from tissues to the lungs and for maintaining the acid-base balance in the body. While $H_2CO_3$ spontaneously dissociates into bicarbonate ($HCO_3^-$) and hydrogen ions ($H^+$), the specific chemical formed directly via the enzymatic action of CA is carbonic acid. **2. Why Incorrect Options are Wrong:** * **Option A:** The formation of $CO_2$ from elemental carbon and oxygen is a combustion process, not a biological enzymatic reaction. * **Option C:** The dissociation of carbonic acid into bicarbonate and $H^+$ is a spontaneous ionic dissociation that occurs rapidly without the need for an enzyme, although CA facilitates the overall equilibrium. * **Option D:** Hydrochloric acid ($HCl$) is secreted by parietal cells in the stomach. While CA is involved in providing the $H^+$ ions for this process, it does not directly synthesize $HCl$. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Zinc Cofactor:** Carbonic anhydrase is the classic example of an enzyme requiring **Zinc ($Zn^{2+}$)** for its catalytic activity. * **Isoforms:** CA II is the most active isoform found in RBCs; CA IV is found in the renal brush border. * **Clinical Application:** Carbonic anhydrase inhibitors like **Acetazolamide** are used to treat glaucoma (by reducing aqueous humor production), altitude sickness, and as a weak diuretic (acting on the proximal convoluted tubule). * **Bohr Effect:** CA plays a crucial role in the Bohr effect by facilitating the production of $H^+$ ions, which decrease hemoglobin's affinity for oxygen in peripheral tissues.

Question 17: A junior doctor had difficulty in determining the base deficit/excess for blood in a given patient. An experienced senior resident advised a quick method to determine the acid-base composition of blood based on pCO2. Which of the following is the likely method he suggested to predict acid-base composition of blood?

A. Redford nomogram
B. Dübios nomogram
C. Goldman constant field equation
D. Siggard-Andersen nomogram (Correct Answer)

Explanation: **Explanation:** The **Siggard-Andersen nomogram** is the standard clinical tool used to determine the acid-base status of blood. It utilizes the relationship between **pH, pCO₂, and base excess/deficit**. By plotting the measured pH and pCO₂ on the nomogram, a clinician can derive the base excess, which quantifies the metabolic component of an acid-base disturbance. This is essential for distinguishing between respiratory and metabolic causes of acidosis or alkalosis. **Analysis of Incorrect Options:** * **Redford nomogram (Option A):** This is a distractor; there is no widely recognized "Redford nomogram" in clinical biochemistry. * **Dübios nomogram (Option B):** This is used to estimate **Body Surface Area (BSA)** based on a patient’s height and weight. It is commonly used for calculating drug dosages (e.g., chemotherapy) and cardiac index. * **Goldman constant field equation (Option C):** Also known as the Goldman-Hodgkin-Katz equation, it is used in cell physiology to determine the **resting membrane potential** of a cell by considering the permeability and concentration gradients of multiple ions (Na⁺, K⁺, Cl⁻). **Clinical Pearls for NEET-PG:** * **Base Excess (BE):** Defined as the amount of strong acid or base required to return 1 liter of blood to a pH of 7.40 at a pCO₂ of 40 mmHg. Normal range is **-2 to +2 mEq/L**. * **Negative Base Excess:** Indicates **Metabolic Acidosis** (Base Deficit). * **Positive Base Excess:** Indicates **Metabolic Alkalosis**. * The **Henderson-Hasselbalch equation** is the mathematical foundation for these relationships: $pH = pKa + \log([HCO_3^-] / [0.03 \times pCO_2])$.

Question 18: What is the equivalent weight of Ca2+ ion?

A. 10 grams
B. 20 grams (Correct Answer)
C. 30 grams
D. 40 grams

Explanation: ### Explanation The **Equivalent Weight** of an ion is defined as its atomic weight divided by its valence (charge). This concept is crucial in biochemistry for calculating electrolyte concentrations in milliequivalents (mEq/L), which is the standard unit for clinical fluid management. **The Formula:** $$\text{Equivalent Weight} = \frac{\text{Atomic Weight}}{\text{Valence}}$$ **Calculation for Ca²⁺:** 1. **Atomic Weight of Calcium:** ~40 grams. 2. **Valence (Charge):** Calcium exists as a divalent cation ($Ca^{2+}$), so its valence is 2. 3. **Calculation:** $40 / 2 = \mathbf{20\text{ grams}}$. --- ### Analysis of Options: * **Option B (20g) is Correct:** As calculated above, one equivalent of Calcium is 20g. Therefore, 1 mEq of $Ca^{2+}$ is 20 mg. * **Option D (40g) is Incorrect:** This represents the **Atomic Weight** (or Molar Mass) of Calcium. This would only be the equivalent weight for a monovalent ion (like $Na^+$ or $K^+$). * **Options A & C (10g & 30g) are Incorrect:** These values do not correspond to the stoichiometry of Calcium. --- ### NEET-PG Clinical Pearls: * **mEq/L vs mg/dL:** To convert Calcium from mg/dL to mEq/L: $\text{mEq/L} = \frac{\text{mg/dL} \times 10 \times \text{Valence}}{\text{Atomic Weight}}$. * **High-Yield Valences:** * **Monovalent (Valence 1):** $Na^+$, $K^+$, $Cl^-$, $HCO_3^-$. (Equivalent Weight = Atomic Weight). * **Divalent (Valence 2):** $Ca^{2+}$, $Mg^{2+}$. (Equivalent Weight = Atomic Weight / 2). * **Normal Range:** Normal serum calcium is 8.5–10.5 mg/dL. Roughly half is ionized (physiologically active), while the rest is bound to albumin or complexed with anions. Always check **Corrected Calcium** in patients with hypoalbuminemia.

Question 19: Increased anion gap acidosis is seen in which of the following conditions?

A. Diabetic Ketoacidosis (DKA) (Correct Answer)
B. Aspirin Toxicity
C. Alcoholic Ketoacidosis
D. Organic Aciduria

Explanation: **Explanation:** **High Anion Gap Metabolic Acidosis (HAGMA)** occurs when there is an accumulation of unmeasured anions in the blood. The Anion Gap (AG) is calculated as: $AG = Na^+ - (Cl^- + HCO_3^-)$. The normal range is 8–12 mEq/L. **Why Diabetic Ketoacidosis (DKA) is the Correct Answer:** In DKA, insulin deficiency leads to the breakdown of fatty acids into ketone bodies: **Acetoacetate** and **$\beta$-hydroxybutyrate**. These are unmeasured organic anions that consume bicarbonate ($HCO_3^-$) to buffer the excess $H^+$ ions. As bicarbonate levels drop without a corresponding increase in chloride, the anion gap increases. **Analysis of Other Options:** While the question asks for "which of the following," it is important to note that **all four options (A, B, C, and D) actually cause a High Anion Gap Metabolic Acidosis.** However, in the context of standard NEET-PG clinical vignettes, **DKA** is the classic, most frequently tested prototype for HAGMA. * **Aspirin Toxicity:** Salicylates are unmeasured anions and also interfere with oxidative phosphorylation, leading to lactic acid buildup. * **Alcoholic Ketoacidosis:** Results from accumulation of $\beta$-hydroxybutyrate due to ethanol-induced starvation. * **Organic Aciduria:** Inherited metabolic disorders (e.g., Methylmalonic acidemia) lead to the accumulation of various organic acids. **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):** Also called Hyperchloremic acidosis. Key causes include **Diarrhea** and **Renal Tubular Acidosis (RTA)**. * **Goldman’s Formula:** Used to calculate the expected $pCO_2$ in metabolic acidosis to check for respiratory compensation.

Question 20: Hypomagnesemia is associated with which of the following electrolyte imbalances?

A. Hypercalcemia
B. Hypokalemia (Correct Answer)
C. Hyperkalemia
D. Hyperphosphatemia

Explanation: **Explanation:** **Why Hypokalemia is the correct answer:** Hypomagnesemia is a frequent cause of refractory hypokalemia. The underlying mechanism involves the **ROMK (Renal Outer Medullary Potassium) channels** in the distal nephron. Under normal physiological conditions, intracellular magnesium acts as a "plug," inhibiting the excessive efflux of potassium through these channels. When magnesium levels are low, this inhibitory effect is lost, leading to uncontrolled potassium secretion into the urine. Consequently, hypokalemia cannot be corrected until the magnesium deficiency is addressed. **Analysis of Incorrect Options:** * **A. Hypercalcemia:** Incorrect. Hypomagnesemia is actually associated with **hypocalcemia**. Low magnesium levels impair the release of Parathyroid Hormone (PTH) and induce skeletal resistance to PTH action. * **C. Hyperkalemia:** Incorrect. As explained above, low magnesium promotes potassium wasting, leading to low serum potassium, not high. * **D. Hyperphosphatemia:** Incorrect. There is no direct causal link between isolated hypomagnesemia and elevated phosphate levels. **Clinical Pearls for NEET-PG:** * **Refractory Hypokalemia:** Always check magnesium levels in patients whose potassium levels do not rise despite adequate supplementation. * **ECG Changes:** Hypomagnesemia presents similarly to hypokalemia (prolonged QT, flattened T waves, and presence of U waves). * **Torsades de Pointes:** Magnesium sulfate is the drug of choice for this specific arrhythmia, often triggered by electrolyte imbalances. * **Digoxin Toxicity:** Hypomagnesemia predisposes patients to digoxin toxicity, similar to hypokalemia.

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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