Acid-Base and Electrolyte Balance Practice Questions

Q: Protein acts as a buffer due to which property?

Amphipathic (amphoteric in nature). **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.

Q: 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?

Thiamine. **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**.

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

Renal tubular acidosis. 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.

Q: What does the delta gap compare?

Change in anion gap with bicarbonate. **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).

Q: All of the following are causes of hypocalcemia, EXCEPT?

Vitamin D intoxication. **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).

Q: 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?

Sodium, glucose, and urea. ### 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.

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

Metabolic acidosis with an increased anion gap. ### 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.

Question 1

Protein acts as a buffer due to which property?

Accepted Answer

Amphipathic (amphoteric in nature)

Answer

Colloid

Answer

Basic

Answer

Acidic

Question 2

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?

Accepted Answer

Thiamine

Answer

Folic acid

Answer

Niacin

Answer

Riboflavin

Question 3

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?

Accepted Answer

150 mEq

Answer

300 mEq

Answer

450 mEq

Answer

600 mEq

Question 4

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

Accepted Answer

Hyponatremia

Answer

Metabolic acidosis

Answer

Hypothermia

Answer

Hypocalcemia

Question 5

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?

Accepted Answer

Cystathionine beta-synthase

Answer

S-Adenosylhomocysteine hydrolase

Answer

Methionine synthase

Answer

N5,N10-Methylenetetrahydrofolate reductase

Question 6

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

Accepted Answer

Renal tubular acidosis

Answer

Ethyleneglycol poisoning

Answer

Diabetic ketoacidosis

Answer

Lactic acidosis

Question 7

What does the delta gap compare?

Accepted Answer

Change in anion gap with bicarbonate

Answer

Change in anion gap with chloride only

Answer

Change in anion gap with sodium and potassium

Answer

Change in anion gap with sodium only

Question 8

All of the following are causes of hypocalcemia, EXCEPT?

Accepted Answer

Vitamin D intoxication

Answer

Acute pancreatitis

Answer

Chronic renal failure

Answer

Hypoparathyroidism

Question 9

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?

Accepted Answer

Sodium, glucose, and urea

Answer

Sodium, potassium, chloride, and bicarbonate

Answer

Sodium, potassium, urea, and hemoglobin

Answer

Sodium, albumin, urea, and glucose

Question 10

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

Accepted Answer

Metabolic acidosis with an increased anion gap

Answer

Metabolic alkalosis with a normal anion gap

Answer

Metabolic alkalosis

Answer

Respiratory alkalosis

Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance — MCQs

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