Acid-Base and Electrolyte Balance Practice Questions

Q: Increased anion gap is seen in:

Lactic acidosis. **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.

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

Lung. **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).

Q: Why is bicarbonate considered an ideal buffer in blood?

It is present in a high concentration.. **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.

Q: 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?

Siggard-Andersen nomogram. ### 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**.

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

15 mEq/L. **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).

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

Hypoaldosteronism. **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.

Q: Non-anion gap acidosis is seen in all except:

Starvation. **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.

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

Ketone bodies. **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).

Question 1

Increased anion gap is seen in:

Accepted Answer

Lactic acidosis

Answer

Enterocutaneous fistula

Answer

Ileostomy fistula

Answer

All of the above

Question 2

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

Accepted Answer

Aldosterone

Answer

Cortisol

Answer

Androstenedione

Answer

Testosterone

Question 3

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

Accepted Answer

Lung

Answer

Kidney

Answer

Liver

Answer

Skin

Question 4

Why is bicarbonate considered an ideal buffer in blood?

Accepted Answer

It is present in a high concentration.

Answer

Its pKa is equal to the physiological pH.

Answer

It consists of a weak acid and a weak base.

Answer

It is easy to measure.

Question 5

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?

Accepted Answer

Siggard-Andersen nomogram

Answer

Redford normogram

Answer

Dübios normogram

Answer

Goldman constant field equation

Question 6

Anion gap is increased by all except?

Accepted Answer

Iron

Answer

Ethylene glycol

Answer

Methanol

Answer

Salicylates

Question 7

The anion gap in a healthy individual is around ___.

Accepted Answer

15 mEq/L

Answer

5 mEq/L

Answer

20 mEq/L

Answer

25 mEq/L

Question 8

Metabolic acidosis is caused by which of the following conditions?

Accepted Answer

Hypoaldosteronism

Answer

Hypovolemia

Answer

Hypokalemia

Answer

Hypocalcemia

Question 9

Non-anion gap acidosis is seen in all except:

Accepted Answer

Starvation

Answer

Diarrhea

Answer

NSAIDs

Answer

Renal acidosis

Question 10

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

Accepted Answer

Ketone bodies

Answer

Glycogen

Answer

Glucose

Answer

Sphingolipids

Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance — MCQs

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