Acid-Base and Electrolyte Balance — MCQs

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

Question 91: Increased anion gap is seen in:

A. Enterocutaneous fistula
B. Ileostomy fistula
C. Lactic acidosis (Correct Answer)
D. All of the above

Explanation: **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. A normal anion gap is typically **8–12 mEq/L**. An increased anion gap occurs when there is an accumulation of unmeasured organic acids (like lactate or ketoacids) or a decrease in unmeasured cations. **1. Why Lactic Acidosis is Correct:** In lactic acidosis, excess lactic acid dissociates into $H^+$ and lactate. The $H^+$ ions are buffered by bicarbonate ($HCO_3^-$), leading to a decrease in serum bicarbonate levels. Since the lactate anion is "unmeasured" in the standard formula, the gap between measured cations and anions widens, resulting in a **High Anion Gap Metabolic Acidosis (HAGMA).** **2. Why Other Options are Incorrect:** * **Enterocutaneous and Ileostomy Fistulas:** These conditions involve the direct loss of bicarbonate-rich fluids from the lower GI tract. To maintain electrical neutrality, the kidneys retain chloride ($Cl^-$) to replace the lost bicarbonate. This results in a **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as hyperchloremic metabolic acidosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA (MUDPILES):** **M**ethanol, **U**remia, **D**iabetic Ketoacidosis, **P**araldehyde/Propylene Glycol, **I**soniazid/Iron, **L**actic Acidosis, **E**thylene Glycol, **S**alicylates. * **Mnemonic for NAGMA (USED CARP):** **U**reterosigmoidostomy, **S**aline infusion, **E**ndocrine (Addison’s), **D**iarrhea, **C**arbonic anhydrase inhibitors (Acetazolamide), **A**mmonium chloride, **R**enal tubular acidosis (RTA), **P**ancreatic fistula. * **Albumin Correction:** For every 1 g/dL decrease in serum albumin, the normal anion gap decreases by approximately 2.5 mEq/L. This is a common "trap" in clinical vignettes.

Question 92: 17-alpha hydroxylase is not involved in the pathway for synthesis of which of the following?

A. Cortisol
B. Aldosterone (Correct Answer)
C. Androstenedione
D. Testosterone

Explanation: **Explanation:** The synthesis of steroid hormones occurs in the adrenal cortex and gonads via specific enzymatic pathways. The enzyme **17-alpha hydroxylase** is a critical branch-point enzyme that diverts steroid precursors away from the mineralocorticoid pathway and toward the glucocorticoid and androgen pathways. **Why Aldosterone is the correct answer:** Aldosterone is a mineralocorticoid synthesized in the **Zona Glomerulosa** of the adrenal cortex. This zone lacks the enzyme 17-alpha hydroxylase. Therefore, progesterone is converted to 11-deoxycorticosterone (by 21-hydroxylase) rather than 17-hydroxyprogesterone. Because 17-alpha hydroxylase is absent in this layer, it is not involved in aldosterone synthesis. **Why the other options are incorrect:** * **Cortisol (Option A):** Synthesized in the Zona Fasciculata. 17-alpha hydroxylase is essential here to convert Progesterone to 17-OH Progesterone, the precursor for cortisol. * **Androstenedione & Testosterone (Options C & D):** These are adrenal and gonadal androgens. 17-alpha hydroxylase (specifically its 17,20-lyase activity) is required to convert 17-OH precursors into dehydroepiandrosterone (DHEA) and androstenedione, which eventually forms testosterone. **High-Yield Clinical Pearls for NEET-PG:** * **17-alpha hydroxylase deficiency:** Leads to a decrease in cortisol and sex hormones but an **excess of mineralocorticoids** (specifically 11-deoxycorticosterone). Clinical presentation includes **hypertension, hypokalemia**, and sexual infantilism (delayed puberty/ambiguous genitalia in males). * **Mnemonic for Adrenal Layers (Outer to Inner):** **G**FR – **G**lomerulosa (Mineralocorticoids), **F**asciculata (Glucocorticoids), **R**eticularis (Androgens). "The deeper you go, the sweeter it gets" (Salt $\rightarrow$ Sugar $\rightarrow$ Sex). * **Congenital Adrenal Hyperplasia (CAH):** 21-hydroxylase deficiency is the most common cause (>90%), leading to virilization and salt-wasting.

Question 93: Which of the following organs is NOT primarily involved in maintaining calcium homeostasis?

A. Kidney
B. Lung (Correct Answer)
C. Liver
D. Skin

Explanation: **Explanation:** Calcium homeostasis is a tightly regulated process primarily involving the interplay between the **Parathyroid Hormone (PTH)**, **Calcitriol (Vitamin D3)**, and **Calcitonin**. **Why Lung is the Correct Answer:** The **Lung** is not involved in the metabolic pathways of calcium regulation. Its primary role in acid-base balance is the regulation of $CO_2$ (volatile acid), but it does not possess receptors or enzymatic machinery to alter systemic calcium levels. **Why the other options are incorrect:** * **Skin:** It is the starting point of Vitamin D synthesis. Under the influence of UV-B light, 7-dehydrocholesterol in the skin is converted to Cholecalciferol (Vitamin D3). * **Liver:** It is responsible for the first hydroxylation step in Vitamin D metabolism. It converts Cholecalciferol into **25-hydroxyvitamin D [25(OH)D]** via the enzyme 25-hydroxylase. * **Kidney:** It is the most critical regulatory organ. It performs the second hydroxylation (via 1-alpha-hydroxylase) to produce the active form, **1,25-dihydroxycholecalciferol (Calcitriol)**. It also manages the actual excretion and reabsorption of calcium ions in the renal tubules. **High-Yield Clinical Pearls for NEET-PG:** * **Active Form of Vit D:** Calcitriol (1,25-$(OH)_2D_3$). * **Storage Form of Vit D:** 25-$(OH)D$ (measured to check for deficiency). * **PTH Action:** Increases serum calcium by increasing bone resorption, renal calcium reabsorption, and stimulating 1-alpha-hydroxylase in the kidney. * **Hypocalcemia Sign:** Chvostek's sign (facial twitching) and Trousseau's sign (carpal spasm).

Question 94: Why is bicarbonate considered an ideal buffer in blood?

A. Its pKa is equal to the physiological pH.
B. It is present in a high concentration. (Correct Answer)
C. It consists of a weak acid and a weak base.
D. It is easy to measure.

Explanation: **Explanation:** The bicarbonate buffer system ($H_2CO_3 / HCO_3^-$) is the most important extracellular buffer in the body. While an "ideal" chemical buffer typically has a pKa close to the desired pH, the bicarbonate system is physiologically ideal due to its **high concentration** and its status as an **open system**. 1. **Why Option B is Correct:** In the blood, bicarbonate ($HCO_3^-$) is present in a high concentration (approx. 24 mEq/L). This provides a massive "alkali reserve" to neutralize metabolic acids. Furthermore, it is an open system: the lungs can rapidly regulate $CO_2$ (the acid component) and the kidneys can regulate $HCO_3^-$ (the base component), making it highly efficient despite its pKa. 2. **Why Other Options are Incorrect:** * **Option A:** The pKa of the bicarbonate system is **6.1**. Since physiological pH is **7.4**, the pKa is quite far from the pH. Usually, a buffer works best when $pH = pKa$. * **Option C:** While it does consist of a weak acid ($H_2CO_3$) and its conjugate base ($HCO_3^-$), this is a general definition of *any* buffer and does not explain why this specific system is "ideal" for blood. * **Option D:** Ease of measurement is a clinical convenience, not a physiological reason for its buffering efficiency. **High-Yield NEET-PG Pearls:** * **Henderson-Hasselbalch Equation:** $pH = pKa + \log([HCO_3^-] / [0.03 \times PCO_2])$. * **Ratio:** At pH 7.4, the ratio of $HCO_3^-$ to $H_2CO_3$ is **20:1**. * **Intracellular Buffers:** While bicarbonate is the main ECF buffer, **Proteins (Hemoglobin)** and **Phosphates** are the primary ICF buffers. * **Maximum Buffering Capacity:** Chemically, the phosphate buffer is "better" (pKa 6.8 is closer to 7.4), but bicarbonate wins physiologically due to its high concentration and respiratory/renal regulation.

Question 95: A junior doctor had difficulty in determining the base deficit/excess for blood in a patient. A 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?

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

Explanation: ### Explanation **Correct Answer: D. Siggard-Andersen nomogram** The **Siggard-Andersen nomogram** is a specialized graphical tool used in clinical biochemistry to determine the acid-base status of blood. It allows clinicians to calculate the **Base Excess (BE)** or **Base Deficit** by plotting the relationship between blood pH and $PCO_2$. By knowing any two variables (e.g., pH and $PCO_2$), the third variable (like Bicarbonate or Base Excess) can be derived. This is essential for distinguishing between respiratory and metabolic components of acid-base disturbances. **Analysis of Incorrect Options:** * **A. Redford nomogram:** This is a distractor. While there are various acid-base maps (like the Goldberg or Arbus maps), "Redford" is not a recognized standard in clinical acid-base physiology. * **B. Dübios nomogram:** This is used to calculate **Body Surface Area (BSA)** based on a patient’s height and weight. It is commonly used for dosing chemotherapy or calculating the Cardiac Index, not for acid-base balance. * **C. Goldman constant field equation:** This is a concept in electrophysiology used to determine the **resting membrane potential** of a cell by considering the permeability and concentration gradients of multiple ions (Na+, K+, Cl-). **High-Yield 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 $PCO_2$ of 40 mmHg. Normal range is **-2 to +2 mEq/L**. * **Negative Base Excess** indicates a **Base Deficit**, which is characteristic of Metabolic Acidosis. * **Henderson-Hasselbalch Equation:** The mathematical foundation for acid-base balance: $pH = pKa + \log([HCO_3^-] / [0.03 \times PCO_2])$. * **Anion Gap:** Always calculate this in metabolic acidosis cases ($Na^+ - [Cl^- + HCO_3^-]$); normal is **8–12 mEq/L**.

Question 96: Anion gap is increased by all except?

A. Ethylene glycol
B. Methanol
C. Salicylates
D. Iron (Correct Answer)

Explanation: **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. An increased anion gap metabolic acidosis (HAGMA) occurs when unmeasured anions (like lactate, ketones, or exogenous toxins) accumulate in the blood, consuming bicarbonate. **Why Iron is the correct answer:** While acute iron poisoning *can* cause a high anion gap metabolic acidosis (due to shock and lactic acidosis), it is traditionally **not** listed as a primary cause of HAGMA in standard biochemical classifications compared to the other options. However, in the context of this specific question, there is a common mnemonic confusion. **Iron** is often a "distractor" because it is part of the **MUDPILES** mnemonic (where 'I' stands for **Isoniazid** or **Iron**). In many classical textbooks, Iron is considered a secondary cause, whereas Ethylene glycol, Methanol, and Salicylates are primary, direct causes of increased unmeasured anions. *Note: In some advanced clinical contexts, Iron is included; however, for NEET-PG, if a choice must be made among these, Iron is often the least "direct" cause compared to the metabolic byproducts of the others.* **Analysis of Incorrect Options:** * **Methanol:** Metabolized to **formic acid**, which increases unmeasured anions. * **Ethylene glycol:** Metabolized to **glycolic and oxalic acid**, leading to HAGMA and calcium oxalate crystals in urine. * **Salicylates:** Causes a mixed respiratory alkalosis and HAGMA due to the accumulation of **salicylic acid** and interference with the Krebs cycle (lactic acid). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates). * **Normal Anion Gap:** 8–12 mEq/L. * **Normal Anion Gap Acidosis (NAGMA):** Primarily caused by Diarrhea and Renal Tubular Acidosis (RTA). * **Goldman’s Mnemonic (KUSMALE):** Ketoacidosis, Uremia, Salicylates, Methanol, Aldehydes, Lactate, Ethylene glycol.

Question 97: The anion gap in a healthy individual is around ___.

A. 5 mEq/L
B. 15 mEq/L (Correct Answer)
C. 20 mEq/L
D. 25 mEq/L

Explanation: **Explanation:** The **Anion Gap (AG)** is a calculated parameter used to identify the cause of metabolic acidosis. It represents the difference between measured cations (Sodium) and measured anions (Chloride and Bicarbonate). **The Formula:** $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$ **Why 15 mEq/L is correct:** In a healthy individual, the concentration of measured cations exceeds measured anions. This "gap" is composed of unmeasured anions such as **Albumin** (the most significant contributor), phosphates, sulfates, and organic acids. The normal reference range is typically **8 to 16 mEq/L**. Therefore, **15 mEq/L** is the most accurate value among the choices provided, representing a physiological state. **Analysis of Incorrect Options:** * **A (5 mEq/L):** This is too low. A low anion gap is rare and usually suggests hypoalbuminemia (loss of the primary unmeasured anion) or multiple myeloma (increase in unmeasured cationic IgG). * **C & D (20 & 25 mEq/L):** These values represent a **High Anion Gap Metabolic Acidosis (HAGMA)**. This occurs when pathological unmeasured anions accumulate, such as lactate (sepsis), ketones (DKA), or exogenous toxins (methanol, ethylene glycol). **High-Yield Clinical Pearls for NEET-PG:** 1. **Albumin Correction:** For every 1 g/dL decrease in serum albumin below 4 g/dL, the "normal" anion gap decreases by approximately **2.5 mEq/L**. 2. **MUDPILES:** The classic mnemonic for HAGMA causes (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). 3. **Normal Anion Gap Metabolic Acidosis (NAGMA):** Also called hyperchloremic acidosis; common causes include diarrhea and Renal Tubular Acidosis (RTA).

Question 98: Metabolic acidosis is caused by which of the following conditions?

A. Hypovolemia
B. Hypokalemia
C. Hypocalcemia
D. Hypoaldosteronism (Correct Answer)

Explanation: **Explanation:** **Hypoaldosteronism** (Option D) is the correct answer because aldosterone plays a critical role in the renal regulation of acid-base balance. In the distal convoluted tubule and collecting ducts, aldosterone stimulates the secretion of **Hydrogen ions (H⁺)** via α-intercalated cells and **Potassium ions (K⁺)** via principal cells. A deficiency in aldosterone (as seen in Addison’s disease or Type 4 Renal Tubular Acidosis) leads to the retention of H⁺ and K⁺. This results in **Normal Anion Gap Metabolic Acidosis** accompanied by **hyperkalemia**. **Why the other options are incorrect:** * **Hypovolemia (A):** Severe volume depletion typically leads to **Metabolic Alkalosis** (Contraction Alkalosis). As the body attempts to retain sodium and water, it increases bicarbonate reabsorption and H⁺ secretion (via the RAAS pathway). * **Hypokalemia (B):** Low potassium levels cause an intracellular shift of H⁺ ions and increased renal H⁺ secretion (to conserve K⁺), leading to **Metabolic Alkalosis**. Note: "Hypokalemic, hypochloremic metabolic alkalosis" is a classic board presentation. * **Hypocalcemia (C):** While calcium imbalances affect neuromuscular excitability and cardiac conduction, they do not directly cause metabolic acidosis. **NEET-PG High-Yield Pearls:** * **Aldosterone's Rule:** "Saves Sodium, Spills Potassium and Hydrogen." * **Type 4 RTA:** The most common cause of metabolic acidosis associated with hypoaldosteronism or aldosterone resistance. * **Hyperkalemia vs. Acidosis:** Acidosis generally causes hyperkalemia (H⁺ enters cells, K⁺ exits), but in hypoaldosteronism, the hyperkalemia is a primary result of the hormone deficiency itself.

Question 99: Non-anion gap acidosis is seen in all except:

A. Diarrhea
B. NSAIDs
C. Renal acidosis
D. Starvation (Correct Answer)

Explanation: **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. The normal range is 8–12 mEq/L. **Why Starvation is the correct answer:** Starvation leads to **High Anion Gap Metabolic Acidosis (HAGMA)**. During prolonged fasting, the body shifts to fatty acid oxidation, resulting in the overproduction of ketone bodies (acetoacetate and $\beta$-hydroxybutyrate). These are unmeasured anions that increase the anion gap while consuming bicarbonate buffers. **Analysis of Incorrect Options (Causes of Normal Anion Gap Metabolic Acidosis - NAGMA):** In NAGMA, the loss of $HCO_3^-$ is compensated by a reciprocal increase in $Cl^-$ (Hyperchloremic acidosis), keeping the gap normal. * **Diarrhea:** The most common cause of NAGMA. It involves the direct loss of bicarbonate-rich intestinal secretions. * **NSAIDs:** These can cause Type IV Renal Tubular Acidosis (RTA) by inducing a state of hyporeninemic hypoaldosteronism, leading to NAGMA. * **Renal Acidosis (RTA):** Renal Tubular Acidosis (Types I, II, and IV) are classic causes of NAGMA due to either failure to reabsorb $HCO_3^-$ or failure to excrete $H^+$. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA/Starvation, 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:** If the question mentions "Hyperchloremic Acidosis," always look for NAGMA causes.

Question 100: Acidosis occurs in diabetes and starvation due to which of the following?

A. Glycogen
B. Ketone bodies (Correct Answer)
C. Glucose
D. Sphingolipids

Explanation: **Explanation:** The correct answer is **Ketone bodies**. **1. Why Ketone Bodies are Correct:** In both **Diabetes Mellitus (Type 1)** and **Starvation**, there is a relative or absolute deficiency of insulin. This leads to increased lipolysis in adipose tissue, releasing free fatty acids into the blood. These fatty acids undergo $\beta$-oxidation in the liver to form Acetyl-CoA. Due to the depletion of oxaloacetate (diverted for gluconeogenesis), Acetyl-CoA enters the ketogenic pathway to produce **Acetoacetate** and **$\beta$-hydroxybutyrate**. These are organic acids that dissociate at physiological pH, releasing $H^+$ ions into the bloodstream. This leads to a decrease in blood pH and bicarbonate levels, resulting in **High Anion Gap Metabolic Acidosis (HAGMA)**. **2. Why Other Options are Incorrect:** * **Glycogen:** This is a storage polysaccharide. Its breakdown (glycogenolysis) yields glucose-1-phosphate, which does not contribute to acid production. * **Glucose:** While hyperglycemia is a hallmark of diabetes, glucose itself is a neutral molecule. Acidosis in diabetes is a result of lipid metabolism (ketogenesis), not the high glucose level itself. * **Sphingolipids:** These are structural components of cell membranes (especially in neural tissue). Their metabolism is unrelated to the acute acid-base disturbances seen in metabolic crises like DKA or starvation. **3. NEET-PG High-Yield Pearls:** * **The Three Ketone Bodies:** Acetoacetate, $\beta$-hydroxybutyrate, and Acetone (Note: Acetone is non-acidic and excreted via breath, giving the "fruity odor"). * **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Nitroprusside Test (Rothera’s):** Detects Acetoacetate and Acetone, but **fails** to detect $\beta$-hydroxybutyrate. * **Anion Gap:** DKA is a classic cause of High Anion Gap Metabolic Acidosis (Mnemonic: MUDPILES).

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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