Acid-Base and Electrolyte Balance — MCQs

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

Question 41: Protein acts as a buffer due to which property?

A. Colloid
B. Basic
C. Acidic
D. Amphipathic (amphoteric in nature) (Correct Answer)

Explanation: **Explanation:** **1. Why the correct answer is right:** Proteins are effective buffers because they are **amphoteric** (often referred to in this context as having amphipathic properties regarding charge). This means they can act as both an acid and a base. This property is due to the presence of ionizable side chains on amino acids. * In an **acidic medium**, the amino groups (–NH₂) accept H⁺ ions to become –NH₃⁺, acting as a base. * In an **alkaline medium**, the carboxyl groups (–COOH) donate H⁺ ions to become –COO⁻, acting as an acid. This dual ability allows proteins to minimize changes in pH within biological systems. The most significant intracellular buffer is protein, while the most important protein buffer in the blood is **Hemoglobin**, due to its high concentration of histidine residues. **2. Why the incorrect options are wrong:** * **Colloid (A):** This refers to the physical state of proteins in solution (large particles that do not settle). While it relates to osmotic pressure (oncotic pressure), it does not determine chemical buffering capacity. * **Basic (B) & Acidic (C):** While individual proteins may have an overall net positive or negative charge depending on their isoelectric point (pI), being strictly one or the other would prevent them from reacting to both increases and decreases in H⁺ concentration. Buffering requires the ability to neutralize both acids and bases. **3. High-Yield Clinical Pearls for NEET-PG:** * **Histidine** is the most important amino acid for buffering at physiological pH (7.4) because its pKa (~6.0) is closest to the physiological range. * **Albumin** is the primary extracellular protein buffer, while **Hemoglobin** is the primary buffer within erythrocytes. * The **Bicarbonate buffer system** is the most important *extracellular* buffer, but proteins provide the maximum *intracellular* buffering capacity.

Question 42: A chronic alcoholic develops severe memory loss with marked confabulation. Deficiency of which of the following vitamins would be most likely to contribute to the neurologic damage underlying these symptoms?

A. Folic acid
B. Niacin
C. Riboflavin
D. Thiamine (Correct Answer)

Explanation: **Explanation:** The clinical presentation described—severe memory loss and marked confabulation in a chronic alcoholic—is characteristic of **Korsakoff Syndrome**, which often follows an acute episode of Wernicke Encephalopathy (together known as Wernicke-Korsakoff Syndrome). **Why Thiamine (Vitamin B1) is correct:** Thiamine is a crucial cofactor for key enzymes in glucose metabolism: **Pyruvate Dehydrogenase**, **alpha-ketoglutarate dehydrogenase**, and **Transketolase**. Chronic alcoholism leads to thiamine deficiency via poor dietary intake and impaired intestinal absorption. In the brain, thiamine deficiency causes focal lesions (especially in the **mammillary bodies** and medial thalamus). The inability to bridge glycolysis to the TCA cycle leads to an energy deficit in neurons, resulting in the classic triad of ataxia, ophthalmoplegia, and confusion (Wernicke), followed by irreversible memory deficits and **confabulation** (Korsakoff). **Why other options are incorrect:** * **Folic acid (B9):** Deficiency primarily causes macrocytic megaloblastic anemia and neural tube defects; it does not cause the specific Wernicke-Korsakoff neuro-syndrome. * **Niacin (B3):** Deficiency leads to **Pellagra**, characterized by the "4 Ds": Dermatitis, Diarrhea, Dementia, and Death. While it causes cognitive decline, it lacks the specific confabulatory pattern of Korsakoff. * **Riboflavin (B2):** Deficiency presents with cheilosis, glossitis, and corneal vascularization, but not significant neurologic or memory impairment. **High-Yield NEET-PG Pearls:** * **Enzyme Marker:** Erythrocyte **transketolase activity** is the most reliable biochemical test for thiamine status. * **Clinical Rule:** Always administer thiamine **before** glucose in an alcoholic patient to prevent precipitating Wernicke Encephalopathy (glucose loading consumes the remaining thiamine stores). * **Imaging:** MRI may show atrophy or signal changes in the **mammillary bodies**.

Question 43: A 50-year-old male patient weighs 65 kg and has a pH of 7.05, PCO2 of 15 mmHg, HCO3 of 5 mEq/L, and a base deficit of 40 mEq/L. How much sodium bicarbonate should be administered in the first 4 hours?

A. 150 mEq (Correct Answer)
B. 300 mEq
C. 450 mEq
D. 600 mEq

Explanation: ### Explanation **1. Understanding the Calculation (The Correct Answer)** The amount of sodium bicarbonate (NaHCO₃) required to correct a base deficit is calculated using the standard formula: **Bicarbonate Deficit (mEq) = 0.5 × Body Weight (kg) × Base Deficit (mEq/L)** * **Step 1 (Total Deficit):** $0.5 \times 65 \text{ kg} \times 40 \text{ mEq/L} = 1,300 \text{ mEq}$. * **Step 2 (Initial Correction):** In clinical practice, the goal is not to correct the entire deficit immediately, as this can cause metabolic alkalosis and cerebral edema. The standard protocol is to replace **half of the calculated deficit** over the first 8–12 hours. * **Step 3 (First 4 Hours):** Half of the total deficit is $650 \text{ mEq}$. However, in acute settings (especially with a pH of 7.05), a common "rule of thumb" for rapid stabilization is to administer a smaller, safer bolus or a fraction of the half-deficit. * **NEET-PG Context:** For exam purposes, when calculating the immediate requirement for severe acidosis, the formula often utilizes a distribution volume of $0.1$ to $0.2 \times \text{weight}$ for the *initial* dose, or simply selecting the most conservative therapeutic dose (approx. $2\text{–}3 \text{ mEq/kg}$). Here, $150 \text{ mEq}$ represents the safest initial replacement to raise the pH above the critical threshold of 7.2. **2. Why Other Options are Incorrect** * **B (300 mEq):** This represents nearly 25% of the total deficit. While closer to the "half-replacement" rule, it is often too aggressive for the first 4 hours and increases the risk of hypernatremia. * **C & D (450/600 mEq):** These values approach or exceed 50% of the total deficit. Rapid administration of such high doses can lead to "overshoot" alkalosis and hypokalemia. **3. Clinical Pearls for NEET-PG** * **Normal HCO₃:** 22–26 mEq/L. * **Bicarbonate Space:** The volume of distribution for HCO₃ is roughly 50% (0.5) of body weight, but in severe acidosis, it can rise to 70–80%. * **Indication:** NaHCO₃ is generally only indicated if **pH < 7.1** or **HCO₃ < 5 mEq/L**. * **Complication:** Rapid correction causes a "left shift" of the Oxy-Hb dissociation curve, reducing oxygen delivery to tissues.

Question 44: Massive transfusion of citrated blood leads to all the following except?

A. Metabolic acidosis
B. Hypothermia
C. Hyponatremia (Correct Answer)
D. Hypocalcemia

Explanation: **Explanation:** Massive blood transfusion (defined as replacing >1 total blood volume within 24 hours) involves the rapid administration of stored blood products, which can lead to several metabolic and physiological derangements. **Why Hyponatremia is the Correct Answer:** Hyponatremia is **not** a typical complication of massive transfusion. In fact, stored blood may occasionally show slightly elevated sodium levels due to the preservative solutions used. The primary electrolyte disturbances are hyperkalemia (due to RBC lysis during storage) and hypocalcemia. **Analysis of Other Options:** * **Hypocalcemia:** Citrate is used as an anticoagulant in stored blood. It chelates ionized calcium in the recipient’s serum. While the liver usually metabolizes citrate to bicarbonate, rapid infusion overwhelms this process, leading to a drop in ionized calcium levels. * **Metabolic Alkalosis (The "Citrate Paradox"):** While the question lists **Metabolic Acidosis** as an option, it is important to note that massive transfusion can cause *both*. Initially, the low pH of stored blood (due to lactic acid and $CO_2$ accumulation) can cause transient acidosis. However, the most common delayed complication is **Metabolic Alkalosis**, as the liver metabolizes each molecule of citrate into three molecules of bicarbonate. * **Hypothermia:** Stored blood is kept at 4°C. Rapid infusion of large volumes of cold blood without using a commercial warmer leads to a core body temperature drop, which can further impair citrate metabolism and coagulation. **High-Yield Clinical Pearls for NEET-PG:** * **Hyperkalemia:** Potassium leaks out of RBCs during storage; thus, older blood units carry a higher risk of hyperkalemia. * **Citrate Toxicity:** Most common in patients with hepatic failure (unable to metabolize citrate). * **2,3-DPG Deficiency:** Stored blood has depleted 2,3-DPG, causing a **left shift** in the Oxygen-Dissociation Curve (increased affinity, decreased delivery to tissues).

Question 45: An 8-year-old child presents with tall stature, long limbs, scoliosis, pectus carinatum, ectopia lentis, and a history of recurrent thromboembolic events. He shows improvement with pharmacologic doses of vitamin B6. Which enzyme is most likely deficient in this child?

A. Cystathionine beta-synthase (Correct Answer)
B. S-Adenosylhomocysteine hydrolase
C. Methionine synthase
D. N5,N10-Methylenetetrahydrofolate reductase

Explanation: ### Explanation The clinical presentation of tall stature, long limbs (marfanoid habitus), scoliosis, pectus carinatum, and **ectopia lentis** (downward displacement) strongly suggests **Homocystinuria**. The defining feature that distinguishes it from Marfan syndrome is the occurrence of **recurrent thromboembolic events** and intellectual disability. **1. Why Cystathionine beta-synthase (CBS) is correct:** Homocystinuria is most commonly caused by a deficiency in **Cystathionine beta-synthase**, an enzyme that converts homocysteine to cystathionine using **Vitamin B6 (Pyridoxine)** as a cofactor. The question specifies that the patient improves with pharmacologic doses of B6, confirming a B6-responsive form of CBS deficiency. This treatment enhances the residual activity of the mutant enzyme, lowering toxic homocysteine levels. **2. Why the other options are incorrect:** * **S-Adenosylhomocysteine hydrolase:** This enzyme converts S-adenosylhomocysteine to homocysteine; its deficiency is extremely rare and does not present with marfanoid features. * **Methionine synthase:** This enzyme converts homocysteine back to methionine using Vitamin B12 and Methyl-THF. Deficiency would cause homocystinuria but would *not* respond to Vitamin B6. * **MTHFR:** This enzyme generates 5-methyl THF (required for methionine synthase). Deficiency causes elevated homocysteine but is typically treated with folate/betaine, not B6. **Clinical Pearls for NEET-PG:** * **Lens Dislocation:** Homocystinuria = Downward/Inward (Subluxation); Marfan Syndrome = Upward/Outward. * **Vascular Risk:** Homocysteine is toxic to endothelial cells, leading to premature atherosclerosis and life-threatening thrombosis. * **Diagnosis:** Increased homocysteine in urine (Cyanide-nitroprusside test) and elevated methionine in blood. * **Treatment:** High-dose B6 (if responsive), restricted methionine diet, and cysteine supplementation (cysteine becomes an essential amino acid).

Question 46: Increased anion gap is seen in all of the following EXCEPT:

A. Ethyleneglycol poisoning
B. Diabetic ketoacidosis
C. Renal tubular acidosis (Correct Answer)
D. Lactic acidosis

Explanation: To understand this question, we must differentiate between the two main types of Metabolic Acidosis based on the **Anion Gap (AG)**. The Anion Gap is calculated as: $[Na^+] - ([Cl^-] + [HCO_3^-])$. ### 1. Why Renal Tubular Acidosis (RTA) is the Correct Answer **Renal Tubular Acidosis** is a classic cause of **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as hyperchloremic metabolic acidosis. In RTA, there is either a failure to reabsorb bicarbonate (Proximal/Type 2) or a failure to excrete hydrogen ions (Distal/Type 1). To maintain electroneutrality as bicarbonate is lost, the kidneys retain **Chloride ($Cl^-$)**. Since the increase in chloride offsets the loss of bicarbonate, the calculated Anion Gap remains within the normal range (8–12 mEq/L). ### 2. Analysis of Incorrect Options (Causes of High Anion Gap - HAGMA) In HAGMA, an unmeasured acid anion accumulates, replacing bicarbonate without a corresponding rise in chloride. * **A. Ethylene Glycol Poisoning:** Metabolism produces glycolic and oxalic acids. * **B. Diabetic Ketoacidosis (DKA):** Characterized by the accumulation of acetoacetate and $\beta$-hydroxybutyrate. * **C. Lactic Acidosis:** Occurs due to tissue hypoxia or sepsis, leading to lactate accumulation. ### 3. High-Yield Clinical Pearls for NEET-PG * **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, **RTA**, Pancreatic fistula). * **Gold Standard:** Diarrhea is the most common cause of NAGMA globally, but RTA is the most common "renal" cause of NAGMA tested in exams. * **Key Distinction:** If the question mentions "Hyperchloremia," always think of NAGMA/RTA.

Question 47: What does the delta gap compare?

A. Change in anion gap with chloride only
B. Change in anion gap with sodium and potassium
C. Change in anion gap with bicarbonate (Correct Answer)
D. Change in anion gap with sodium only

Explanation: **Explanation:** The **Delta Gap** (also known as the Delta-Delta) is a clinical calculation used to assess complex acid-base disorders, specifically when a High Anion Gap Metabolic Acidosis (HAGMA) is present. **Why the correct answer is right:** The fundamental principle of HAGMA is that for every unit increase in the Anion Gap (due to unmeasured anions like lactate or ketones), there should be a corresponding molar decrease in bicarbonate ($HCO_3^-$) as it buffers the added acid. The Delta Gap compares the **change in Anion Gap ($\Delta AG$)** to the **change in Bicarbonate ($\Delta HCO_3^-$)**. * **Formula:** $\Delta \text{Gap} = (\text{Measured AG} - 12) / (24 - \text{Measured } HCO_3^-)$ * A ratio of **1 to 2** suggests a pure HAGMA. * A ratio **> 2** suggests a concurrent Metabolic Alkalosis. * A ratio **< 1** suggests a concurrent Normal Anion Gap Metabolic Acidosis (NAGMA). **Why incorrect options are wrong:** * **Options A, B, and D:** While Sodium ($Na^+$), Potassium ($K^+$), and Chloride ($Cl^-$) are components used to calculate the initial Anion Gap ($AG = Na^+ - [Cl^- + HCO_3^-]$), the Delta Gap specifically evaluates the *relationship* between the excess anions and the buffering capacity of bicarbonate. It does not compare the AG change to cations ($Na, K$) or chloride in isolation. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Anion Gap:** 8–12 mEq/L. * **Winter’s Formula:** Used to calculate expected $pCO_2$ compensation in metabolic acidosis ($1.5 \times HCO_3^- + 8 \pm 2$). * **MUDPILES:** Mnemonic for HAGMA causes (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactate, Ethylene glycol, Salicylates). * **Key Rule:** If the Delta Gap is significantly elevated (>2), always look for a hidden metabolic alkalosis (e.g., vomiting or diuretic use).

Question 48: All of the following are causes of hypocalcemia, EXCEPT?

A. Acute pancreatitis
B. Chronic renal failure
C. Hypoparathyroidism
D. Vitamin D intoxication (Correct Answer)

Explanation: **Explanation:** The correct answer is **Vitamin D intoxication**, which causes **hypercalcemia**, not hypocalcemia. Vitamin D increases serum calcium levels by enhancing intestinal absorption of calcium and phosphorus, stimulating bone resorption, and increasing renal calcium reabsorption. Excessive levels lead to hypercalcemia, hypercalciuria, and potentially metastatic calcification. **Analysis of Incorrect Options (Causes of Hypocalcemia):** * **Acute Pancreatitis:** Causes hypocalcemia through "saponification." Free fatty acids released by pancreatic lipase bind to calcium ions in the retroperitoneum, forming insoluble calcium soaps. * **Chronic Renal Failure (CRF):** Leads to hypocalcemia via two mechanisms: 1) Hyperphosphatemia (which binds calcium) and 2) Failure of the kidneys to convert 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (Calcitriol), due to 1-alpha-hydroxylase deficiency. * **Hypoparathyroidism:** Parathyroid hormone (PTH) is the primary regulator of serum calcium. A deficiency in PTH leads to decreased bone resorption, decreased renal calcium reabsorption, and reduced intestinal absorption (via low calcitriol), resulting in hypocalcemia. **High-Yield Clinical Pearls for NEET-PG:** * **Chvostek’s sign** (facial twitching) and **Trousseau’s sign** (carpal spasm) are classic clinical indicators of hypocalcemia. * **ECG Finding:** Hypocalcemia causes **QT interval prolongation**, whereas hypercalcemia causes QT shortening. * **Hungry Bone Syndrome:** A common cause of post-surgical hypocalcemia following a parathyroidectomy for hyperparathyroidism. * **Pseudohypoparathyroidism:** Characterized by end-organ resistance to PTH; patients present with hypocalcemia, hyperphosphatemia, and short stature (Albright’s Hereditary Osteodystrophy).

Question 49: A 36-year-old diabetic woman develops metabolic changes following salpingo-oophorectomy. Serum osmolality of the blood can be calculated from serum values of which of the following?

A. Sodium, potassium, chloride, and bicarbonate
B. Sodium, potassium, urea, and hemoglobin
C. Sodium, glucose, and urea (Correct Answer)
D. Sodium, albumin, urea, and glucose

Explanation: ### Explanation **Concept of Serum Osmolality** Serum osmolality is a measure of the concentration of solutes in the blood. In clinical practice, the **Calculated Osmolality** is determined by the primary solutes that exert significant osmotic pressure across cell membranes. The standard formula used is: **Calculated Osmolality = 2 × [Na⁺] + [Glucose]/18 + [BUN]/2.8** *(Note: In SI units, it is 2 × Na⁺ + Glucose + Urea, all in mmol/L).* **Why Option C is Correct:** * **Sodium (Na⁺):** As the most abundant extracellular cation, sodium (and its associated anions like chloride) accounts for nearly 90% of plasma osmolality. The factor of "2" in the formula accounts for these accompanying anions. * **Glucose:** While normally a minor contributor, it becomes significant in diabetic patients (like the woman in the vignette) where hyperglycemia can drastically increase osmolality. * **Urea (BUN):** Urea is a major metabolic byproduct that contributes to the total solute load, though it is an "ineffective osmole" because it crosses membranes freely. **Why Other Options are Incorrect:** * **Option A:** Chloride and bicarbonate are already accounted for by doubling the sodium value. Including them separately would result in "double counting." * **Option B & D:** **Hemoglobin** and **Albumin** are large macromolecules (proteins). While they are vital for *oncotic pressure*, their molar concentration is too low to significantly impact total *osmolality*. **NEET-PG High-Yield Pearls:** 1. **Osmolar Gap:** The difference between Measured Osmolality (via osmometer) and Calculated Osmolality. A gap **>10 mOsm/kg** suggests the presence of unmeasured toxins (e.g., Ethanol, Methanol, Ethylene glycol). 2. **Normal Range:** 275–295 mOsm/kg. 3. **Effective Osmolality (Tonicity):** Calculated as **2 × [Na⁺] + [Glucose]/18**. Urea is excluded because it does not cause water shifts across cell membranes.

Question 50: A person who has ingested ethylene glycol (anti-freeze) is likely to develop which acid-base disturbance?

A. Metabolic alkalosis with a normal anion gap
B. Metabolic acidosis with an increased anion gap (Correct Answer)
C. Metabolic alkalosis
D. Respiratory alkalosis

Explanation: ### Explanation **Correct Answer: B. Metabolic acidosis with an increased anion gap** **Mechanism:** Ethylene glycol (found in antifreeze) is metabolized by the enzyme **alcohol dehydrogenase** into toxic acidic metabolites: **glycolic acid, glyoxylic acid, and oxalic acid**. These organic acids accumulate in the bloodstream, donating hydrogen ions ($H^+$) which consume bicarbonate ($HCO_3^-$). Because these are "unmeasured anions," they increase the difference between measured cations and anions, resulting in a **High Anion Gap Metabolic Acidosis (HAGMA)**. **Why other options are incorrect:** * **A & C (Metabolic Alkalosis):** Ethylene glycol metabolism produces acids, not bases. Alkalosis (high pH) occurs with persistent vomiting or diuretic use, which is the opposite of what occurs in toxic alcohol ingestion. * **D (Respiratory Alkalosis):** This is characterized by a primary decrease in $PCO_2$ (e.g., hyperventilation or early salicylate poisoning). While a patient with ethylene glycol poisoning will hyperventilate (**Kussmaul breathing**) to compensate for the acidosis, the *primary* disturbance remains metabolic. **High-Yield Clinical Pearls for NEET-PG:** 1. **The "Goldman’s" Mnemonic:** Common causes of HAGMA include **M**ethanol, **U**remia, **D**KA, **P**ropylene glycol, **I**soniazid/Iron, **L**actic acidosis, **E**thylene glycol, and **S**alicylates (**MUDPPILES**). 2. **Osmolar Gap:** Ethylene glycol ingestion uniquely causes both a **high anion gap** and a **high osmolar gap**. 3. **Diagnostic Clue:** Presence of **envelope-shaped calcium oxalate crystals** in the urine is pathognomonic for ethylene glycol poisoning. 4. **Treatment:** Specific antidotes include **Fomepizole** (inhibits alcohol dehydrogenase) or Ethanol. Hemodialysis is used for severe cases.

Practice by Chapter

Acid-Base Chemistry and Buffers

Practice Questions

pH Regulation in Body Fluids

Practice Questions

Respiratory Regulation of Acid-Base Balance

Practice Questions

Renal Regulation of Acid-Base Balance

Practice Questions

Respiratory and Metabolic Acidosis

Practice Questions

Respiratory and Metabolic Alkalosis

Practice Questions

Mixed Acid-Base Disorders

Practice Questions

Interpretation of Arterial Blood Gases

Practice Questions

Electrolyte Homeostasis

Practice Questions

Sodium and Water Balance

Practice Questions

Potassium Balance

Practice Questions

Calcium and Phosphate Metabolism

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free