Acid-Base and Electrolyte Balance Practice Questions

Q: Which buffer system has the highest pKa?

Ammonia. **Explanation** The pKa of a buffer system represents the pH at which the acid and its conjugate base are present in equal concentrations. A higher pKa indicates a weaker acid that holds onto its protons more tightly in alkaline environments. **Why Ammonia is Correct:** The **Ammonia ($NH_3/NH_4^+$) buffer system** has a **pKa of approximately 9.2–9.3**. This is significantly higher than the physiological pH of blood (7.4). In the distal tubule of the kidney, ammonia ($NH_3$) diffuses into the lumen and reacts with secreted $H^+$ ions to form ammonium ($NH_4^+$). Because the pKa is so high, the reaction almost exclusively favors the formation of $NH_4^+$, which is "trapped" in the urine (ion trapping). This makes it the most important system for the excretion of metabolic acids and the regeneration of bicarbonate. **Why the others are incorrect:** * **Phosphate ($HPO_4^{2-}/H_2PO_4^-$):** Has a **pKa of 6.8**. While it is an effective urinary and intracellular buffer because its pKa is close to physiological pH, it is lower than that of ammonia. * **Bicarbonate ($H_2CO_3/HCO_3^-$):** Has a **pKa of 6.1**. Despite having a pKa further from 7.4, it is the most important extracellular buffer because it is an open system (CO₂ regulated by lungs, $HCO_3^-$ by kidneys). * **Intracellular Proteins:** The most important protein buffer is Hemoglobin. The imidazole group of Histidine residues provides a **pKa of approximately 6.0–7.0**. **High-Yield NEET-PG Pearls:** * **Most important ECF buffer:** Bicarbonate system. * **Most important ICF buffer:** Proteins and Phosphate. * **Most important Urinary buffer:** Phosphate (for titratable acidity) and Ammonia (for non-titratable acidity). * **Maximum buffering capacity** occurs when **pH = pKa**.

Q: Which of the following causes hyperchloremic metabolic acidosis?

Diarrhea. **Explanation:** Metabolic acidosis is categorized based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. The normal range is 8–12 mEq/L. **Why Diarrhea is Correct:** Diarrhea causes **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as **hyperchloremic metabolic acidosis**. In the lower GI tract, secretions are rich in bicarbonate ($HCO_3^-$). Profuse diarrhea leads to significant loss of bicarbonate. To maintain electrical neutrality in the extracellular fluid, the kidneys retain Chloride ($Cl^-$) ions to replace the lost negative bicarbonate ions. Since the sum of measured anions ($Cl^- + HCO_3^-$) remains constant, the Anion Gap does not change, but chloride levels rise. **Why Other Options are Incorrect:** * **Ethylene glycol poisoning, Diabetic ketoacidosis (DKA), and Lactic acidosis** are all causes of **High Anion Gap Metabolic Acidosis (HAGMA)**. * In these conditions, metabolic acidosis is caused by the accumulation of "unmeasured" organic acids (e.g., glycolate, beta-hydroxybutyrate, or lactate). As these acids dissociate, the $H^+$ consumes $HCO_3^-$, but the corresponding acid anion (which is unmeasured) increases the Anion Gap. Chloride levels typically remain normal in these scenarios. **High-Yield Clinical Pearls for NEET-PG:** * **NAGMA Mnemonic (USED CARP):** **U**reterosigmoidostomy, **S**aline infusion (large volume), **E**ndocrine (Addison’s), **D**iarrhea, **C**arbonic anhydrase inhibitors (Acetazolamide), **A**mmonium chloride, **R**enal tubular acidosis (RTA), **P**ancreatic fistula. * **HAGMA Mnemonic (MUDPILES):** **M**ethanol, **U**remia, **D**KA, **P**araldehyde, **I**soniazid/Iron, **L**actic acidosis, **E**thylene glycol, **S**alicylates. * **Key Distinction:** If the question mentions "Hyperchloremia," always look for GI losses (Diarrhea) or Renal Tubular Acidosis (RTA).

Q: What is the osmolality of plasma if serum sodium is 135 mEq/L, serum glucose is 120 mg/dL, and BUN is 24 mg/dL?

286. ### Explanation **1. The Correct Answer: C (286 mOsm/kg)** The osmolality of plasma is calculated using the standard formula for **Calculated Serum Osmolality**: $$Osmolality = 2 \times [Na^+] + \frac{Glucose}{18} + \frac{BUN}{2.8}$$ **Step-by-step Calculation:** * **Sodium component:** $2 \times 135 = 270$ (Sodium is doubled to account for associated anions like Chloride and Bicarbonate). * **Glucose component:** $120 / 18 \approx 6.6$ * **BUN component:** $24 / 2.8 \approx 8.5$ * **Total:** $270 + 6.6 + 8.5 = 285.1$ Rounding to the nearest whole number gives **286 mOsm/kg**. **2. Analysis of Incorrect Options:** * **Option A (276):** This value is too low and usually results from neglecting the glucose and BUN contributions or using an incorrect multiplier for sodium. * **Option B (285):** While very close, 286 is the more precise mathematical result when accounting for the decimal values of glucose and BUN. * **Option D (290):** This represents the upper limit of the normal range (275–295 mOsm/kg) but does not match the specific values provided in the question. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Osmolar Gap:** The difference between *measured* osmolality (via osmometer) and *calculated* osmolality. A gap **>10 mOsm/L** suggests the presence of unmeasured osmotically active substances (e.g., Ethanol, Methanol, Ethylene glycol). * **Effective Osmolality (Tonicity):** Calculated as $2 \times [Na^+] + \frac{Glucose}{18}$. Urea is excluded because it is an "ineffective osmole" that freely crosses cell membranes. * **Sodium’s Role:** Sodium and its associated anions contribute nearly 95% of the total plasma osmolality. * **SI Units:** If Glucose and BUN are provided in mmol/L, the formula simplifies to: $2 \times [Na^+] + Glucose + Urea$.

Q: Which of the following conditions are associated with hypophosphatemia?

Renal tubular acidosis. **Explanation:** Hypophosphatemia occurs when there is a shift of phosphate into cells, decreased intestinal absorption, or increased renal excretion. **Why Renal Tubular Acidosis (RTA) is correct:** In both Type 1 (Distal) and Type 2 (Proximal) RTA, there is significant renal phosphate wasting. In **Type 2 RTA (Fanconi Syndrome)**, the proximal tubule fails to reabsorb solutes, directly leading to phosphaturia. In **Type 1 RTA**, the chronic metabolic acidosis causes bone buffering, which releases calcium and phosphate; the resulting hypercalciuria leads to secondary hyperparathyroidism, which further increases renal phosphate excretion, causing hypophosphatemia. **Analysis of Incorrect Options:** * **Pseudohypoparathyroidism:** This is characterized by end-organ resistance to PTH. Since PTH normally promotes phosphate excretion, resistance leads to **hyperphosphatemia** (similar to hypoparathyroidism). * **Rickets:** While Vitamin D deficiency rickets is classically associated with hypophosphatemia, the question asks for the *most* definitive association among options. In the context of NEET-PG, RTA is a high-yield cause of profound renal phosphate loss. (Note: If this were a multiple-choice "select all," Rickets would apply, but RTA is the primary metabolic focus here). * **Respiratory Acidosis:** Acidosis generally causes phosphate to shift **out** of cells into the extracellular fluid, potentially causing hyperphosphatemia. Conversely, respiratory *alkalosis* is a major cause of hypophosphatemia. **High-Yield Clinical Pearls for NEET-PG:** * **Refeeding Syndrome:** The most common cause of severe hypophosphatemia in hospitalized patients (due to insulin-mediated intracellular shift). * **PTH Effect:** PTH is "Phosphaturic" (decreases reabsorption in the proximal tubule). * **Fanconi Syndrome Triad:** Phosphaturia (hypophosphatemia), Glycosuria (with normal blood glucose), and Aminoaciduria.

Q: Which of the following causes normal anion gap metabolic acidosis?

Cholera. **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. **Why Cholera is correct:** Cholera causes severe secretory diarrhea. In the lower gastrointestinal tract, secretions are rich in bicarbonate ($HCO_3^-$). The direct loss of bicarbonate leads to a decrease in the blood pH. To maintain electroneutrality, the kidneys retain Chloride ($Cl^-$) ions. Because the decrease in $HCO_3^-$ is offset by an increase in $Cl^-$, the total anion gap remains within the normal range (8–12 mEq/L). This is why it is also termed **Hyperchloremic Metabolic Acidosis**. **Why the other options are incorrect:** * **B, C, and D (Starvation, Ethylene glycol, Lactic acidosis):** These are all causes of **High Anion Gap Metabolic Acidosis (HAGMA)**. In these conditions, metabolic acidosis occurs due to the accumulation of unmeasured organic acids (Ketoacids in starvation, Glycolic acid in ethylene glycol, and Lactic acid in hypoxia). These acid anions "replace" bicarbonate without a corresponding increase in chloride, thus widening the gap. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for NAGMA (Normal Anion Gap):** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra-chloride, Diarrhea, Carbonic anhydrase inhibitors, Renal tubular acidosis, Pancreatic fistula). * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Key Distinction:** Diarrhea (NAGMA) vs. Vomiting (Metabolic Alkalosis).

Q: Which of the following conditions is NOT associated with an increased anion-gap metabolic acidosis?

COPD. **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. An increased anion gap (HAGMA) occurs when unmeasured acid anions (like lactate, ketones, or exogenous toxins) accumulate in the blood, consuming bicarbonate. **Why COPD is the correct answer:** COPD (Chronic Obstructive Pulmonary Disease) causes **Respiratory Acidosis**, not metabolic acidosis. In COPD, the primary pathology is the failure of the lungs to eliminate $CO_2$, leading to hypercapnia (increased $pCO_2$). While the body may compensate by increasing bicarbonate levels, it does not involve the accumulation of unmeasured acid anions that characterize a high anion gap. **Analysis of Incorrect Options:** * **Shock (Option A):** Leads to tissue hypoxia and anaerobic metabolism, resulting in **Lactic Acidosis**. Lactate is an unmeasured anion that increases the AG. * **Ingestion of Antifreeze (Option B):** Antifreeze contains **Ethylene Glycol**, which is metabolized into glycolic and oxalic acids. These organic acids increase the AG. * **Diabetic Ketoacidosis (Option C):** Results in the overproduction of **$\beta$-hydroxybutyrate and acetoacetate**. These ketoacids consume bicarbonate and increase the AG. **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):** Primarily caused by GI loss of $HCO_3^-$ (Diarrhea) or Renal Tubular Acidosis (RTA). * **Normal AG range:** 8–12 mEq/L. * In **COPD**, the compensation is renal (retention of $HCO_3^-$), which takes 3–5 days to fully manifest.

Q: Which of the following buffer systems is most effective for maintaining a physiological pH of 7.4?

Phosphate buffer (pKa = 6.9). The effectiveness of a buffer system is determined by the **Henderson-Hasselbalch equation**. A buffer is most efficient when its **pKa is closest to the desired pH** of the solution (ideally within ±1 pH unit). At this point, the concentrations of the conjugate base and weak acid are nearly equal, allowing the system to neutralize both added acids and bases effectively. ### Why Phosphate Buffer is Correct The physiological pH of blood is **7.4**. Among the options, the **Phosphate buffer (pKa = 6.9)** has a pKa closest to 7.4. While it is a minor buffer in the plasma due to low concentration, it is the **most potent intracellular buffer** and the primary buffer in **urine**, where the pH is closer to its pKa. ### Analysis of Incorrect Options * **Carbonic acid buffer (pKa = 6.1):** Although its pKa is 1.3 units away from 7.4, it is the most important **extracellular** buffer. Its effectiveness stems not from its pKa, but from being an **"open system"** where $CO_2$ levels can be rapidly adjusted by the lungs and $HCO_3^-$ by the kidneys. * **Glutamate buffer (pKa = 8.7):** The pKa is too high; it would be most effective at a basic pH of 8.7. * **Acetate buffer (pKa = 4.5):** The pKa is too low; it is effective only in highly acidic environments. ### NEET-PG High-Yield Pearls * **Maximum Buffering Capacity:** Occurs when **pH = pKa**. * **Primary Extracellular Buffer:** Bicarbonate/Carbonic acid system. * **Primary Intracellular Buffer:** Proteins (specifically **Histidine** residues due to a pKa of ~6.0) and Phosphate. * **Isohydric Principle:** All buffer systems in a common solution (like plasma) are in equilibrium with the same $[H^+]$; a change in one system affects all others.

Q: All are causes of increased anion gap except?

Glue sniffing. To solve this question, one must distinguish between **High Anion Gap Metabolic Acidosis (HAGMA)** and **Normal Anion Gap Metabolic Acidosis (NAGMA)**. ### **Explanation** The Anion Gap (AG) is calculated as: $[Na^+] - ([Cl^-] + [HCO_3^-])$. A high anion gap occurs when unmeasured anions (like lactate, ketones, or exogenous toxins) accumulate in the blood. **Why "Glue Sniffing" is the correct answer:** Glue sniffing involves the inhalation of **Toluene**. Toluene is metabolized to hippuric acid, which is rapidly excreted by the kidneys. This process leads to a loss of bicarbonate and a compensatory increase in chloride, resulting in **Normal Anion Gap Metabolic Acidosis (NAGMA)**. Additionally, toluene can cause Type 1 Renal Tubular Acidosis (RTA), which is a classic cause of NAGMA. **Why the other options are incorrect:** * **Diabetic Ketoacidosis (DKA):** Accumulation of acetoacetate and beta-hydroxybutyrate (unmeasured anions) leads to HAGMA. * **Starvation:** Prolonged fasting leads to the production of ketone bodies, causing HAGMA. * **Ethylene Glycol Poisoning:** Metabolism of ethylene glycol produces glycolic and oxalic acids, which are unmeasured anions that significantly increase the anion gap. ### **High-Yield Clinical Pearls for NEET-PG** * **Mnemonic for HAGMA (MUDPILES):** **M**ethanol, **U**remia, **D**KA, **P**araldehyde, **I**soniazid/Iron, **L**actic acidosis, **E**thylene glycol, **S**alicylates. * **Mnemonic for NAGMA (HARDUP):** **H**yperalimentation, **A**cetazolamide, **R**enal Tubular Acidosis (RTA), **D**iarrhea, **U**retero-sigmoidostomy, **P**ancreatic fistula. * **Key Distinction:** If a patient presents with metabolic acidosis and a **high osmolar gap**, suspect Methanol or Ethylene glycol poisoning.

Q: Which of the following is a precursor of the molecule shown in the illustration?

Insulin. ***Insulin*** - The molecule shown is **C-peptide**, which is produced during **insulin biosynthesis** when **proinsulin** is cleaved to form insulin and C-peptide. - **Insulin** serves as the direct precursor in this pathway: **Preproinsulin** → **Proinsulin** → **Insulin + C-peptide**, making insulin the correct answer. *Glucagon* - **Glucagon** is synthesized from **proglucagon** through a completely separate biosynthetic pathway in pancreatic **alpha cells**. - It has **opposing metabolic effects** to insulin and does not contribute to **C-peptide formation**. *Somatostatin* - **Somatostatin** is derived from **prosomatostatin** in pancreatic **delta cells** and hypothalamic neurons. - It functions as a **growth hormone inhibitor** and has no role in the **insulin-C-peptide** biosynthetic pathway. *Lipase* - **Pancreatic lipase** is a **digestive enzyme** synthesized as **prolipase** and activated in the small intestine. - It is involved in **fat digestion**, not hormone biosynthesis, and has no connection to **C-peptide formation**.

Question 1

Which buffer system has the highest pKa?

Accepted Answer

Ammonia

Answer

Phosphate

Answer

Bicarbonate

Answer

Intracellular proteins

Question 2

A 27-year-old woman presents to the emergency room with a panic attack. She appears healthy except for tachycardia and a respiratory rate of 30. Electrolytes include calcium 10.0 mg/dL, albumin 4.0 g/dL, phosphorus 0.8 mg/dL, and magnesium 1.5 mEq/L. Arterial blood gases include pH of 7.56, PCO2 21 mm Hg, and PO2 99 mm Hg. Which of the following is the most likely cause of the hypophosphatemia?

Accepted Answer

Respiratory alkalosis with intracellular shift

Answer

Hypomagnesemia

Answer

Hyperparathyroidism

Answer

Poor dietary intake

Question 3

Which of the following causes hyperchloremic metabolic acidosis?

Accepted Answer

Diarrhea

Answer

Ethylene glycol poisoning

Answer

Diabetic ketoacidosis (DKA)

Answer

Lactic acidosis

Question 4

What is the osmolality of plasma if serum sodium is 135 mEq/L, serum glucose is 120 mg/dL, and BUN is 24 mg/dL?

Accepted Answer

286

Answer

276

Answer

285

Answer

290

Question 5

Which of the following conditions are associated with hypophosphatemia?

Accepted Answer

Renal tubular acidosis

Answer

Pseudohypoparathyroidism

Answer

Rickets

Answer

Respiratory acidosis

Question 6

Which of the following causes normal anion gap metabolic acidosis?

Accepted Answer

Cholera

Answer

Starvation

Answer

Ethylene glycol poisoning

Answer

Lactic acidosis

Question 7

Which of the following conditions is NOT associated with an increased anion-gap metabolic acidosis?

Accepted Answer

COPD

Answer

Shock

Answer

Ingestion of antifreeze

Answer

Diabetic ketoacidosis

Question 8

Which of the following buffer systems is most effective for maintaining a physiological pH of 7.4?

Accepted Answer

Phosphate buffer (pKa = 6.9)

Answer

Carbonic acid buffer (pKa = 6.1)

Answer

Glutamate buffer (pKa = 8.7)

Answer

Acetate buffer (pKa = 4.5)

Question 9

All are causes of increased anion gap except?

Accepted Answer

Glue sniffing

Answer

Diabetic ketoacidosis

Answer

Starvation

Answer

Ethylene glycol poisoning

Question 10

Which of the following is a precursor of the molecule shown in the illustration?

Accepted Answer

Insulin

Answer

Glucagon

Answer

Somatostatin

Answer

Lipase

Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance — MCQs

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