Acid-Base and Electrolyte Balance Practice Questions

Q: Which is the most effective buffer system in the blood that is controlled by respiration?

Bicarbonates. **Explanation:** The **Bicarbonate buffer system ($HCO_3^- / CO_2$)** is the most important and effective extracellular buffer in the blood. Its effectiveness stems from being an **"open system."** Unlike other buffers, its components are independently regulated by two major organs: the **lungs** (controlling $CO_2$ via respiration) and the **kidneys** (controlling $HCO_3^-$ excretion and reabsorption). According to the Henderson-Hasselbalch equation, the pH of blood is determined by the ratio of bicarbonate to dissolved $CO_2$. By increasing or decreasing the rate of respiration (ventilation), the body can rapidly adjust $pCO_2$ levels to maintain this ratio, making it the primary respiratory-controlled buffer. **Analysis of Incorrect Options:** * **Hemoglobin (B):** While hemoglobin is a powerful buffer (due to histidine residues) and is the most important buffer **inside erythrocytes**, it is not primarily controlled by respiration; it depends on the oxygenation state (Bohr effect). * **Proteins (C):** Plasma proteins (like albumin) act as buffers in the blood, but their concentration remains relatively static and is not acutely regulated by respiratory changes. * **Phosphates (D):** The phosphate buffer system is crucial **intracellularly** and in the **renal tubules** (where its pKa of 6.8 is close to tubular pH). However, its concentration in the plasma is too low to be the most effective blood buffer. **High-Yield Clinical Pearls for NEET-PG:** * **Normal $HCO_3^- : CO_2$ ratio:** 20:1 (maintains physiological pH of 7.4). * **First line of defense:** Chemical buffers (seconds). * **Second line of defense:** Respiratory system (minutes). * **Third line of defense:** Renal system (hours to days). * **Isohydric Principle:** All buffer systems in the body are in equilibrium with each other; a change in one affects all others.

Q: High anion gap is seen in which of the following conditions?

Lactic acidosis. **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as: $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$. The normal range is 8–12 mEq/L. **1. Why Lactic Acidosis is Correct:** In **Lactic Acidosis**, there is an accumulation of lactate (an unmeasured anion). As lactic acid dissociates, the $H^+$ ions are buffered by $HCO_3^-$, leading to a decrease in bicarbonate levels. Since the lactate anion replaces the consumed bicarbonate, the gap between measured cations and anions increases, resulting in a **High Anion Gap Metabolic Acidosis (HAGMA)**. **2. Analysis of Other Options:** * **Renal Failure (Option A):** While advanced chronic kidney disease (CKD) causes HAGMA due to phosphate and sulfate retention, early-stage renal failure or Renal Tubular Acidosis (RTA) typically presents with a Normal Anion Gap. In the context of this question, Lactic Acidosis is the more classic and definitive example of HAGMA. * **Diarrhea (Option C):** This causes **Normal Anion Gap Metabolic Acidosis (NAGMA)**. There is a direct loss of $HCO_3^-$ from the GI tract, which is compensated by a proportional increase in serum Chloride ($Cl^-$), also known as hyperchloremic acidosis. * **Alcoholism (Option D):** Chronic alcoholism itself doesn't cause HAGMA unless it leads to **Alcoholic Ketoacidosis (AKA)**. Without specifying "Ketoacidosis," it is a less precise answer than Lactic Acidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, INH/Iron, **Lactic Acidosis**, Ethylene Glycol, Salicylates). * **Mnemonic for NAGMA:** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhea, Carbonic anhydrase inhibitors, Addison’s, Renal Tubular Acidosis, Pancreatic fistula). * **Gold Standard:** Always check the "Delta Gap" in HAGMA to rule out mixed acid-base disorders.

Q: All of the following are causes of hypovolemic hyponatremia with urine sodium > 20 mEq/L, EXCEPT?

Vomiting. To approach hyponatremia, one must first assess the **volume status** and then the **urinary sodium (U-Na+)** concentration to differentiate the source of sodium loss. ### **Explanation of the Correct Answer** **B. Vomiting:** In cases of vomiting, sodium is lost via the gastrointestinal tract (extra-renal loss). Because the kidneys are functioning normally, they attempt to compensate for the volume depletion by activating the Renin-Angiotensin-Aldosterone System (RAAS). This leads to maximal sodium reabsorption in the tubules, resulting in a **Urine Sodium 20 mEq/L**: * **A. Renal losses (General):** Includes diuretic use (especially thiazides), which directly increases urinary sodium excretion. * **C. Cerebral Salt Wasting (CSW):** Seen in CNS insults; it involves a primary loss of sodium through the kidneys, leading to hypovolemia and high U-Na+. * **D. Mineralocorticoid deficiency:** Lack of aldosterone (e.g., Addison’s disease) prevents sodium reabsorption in the distal tubule, causing "salt wasting" into the urine. ### **High-Yield NEET-PG Pearls** * **Hypovolemic Hyponatremia Algorithm:** * **U-Na+ 20 mEq/L:** Renal losses (Diuretics, Mineralocorticoid deficiency, CSW, Salt-losing nephropathy). * **CSW vs. SIADH:** Both have high U-Na+, but CSW is **hypovolemic**, while SIADH is **euvolemic**. * **Correction Rate:** Avoid correcting sodium faster than **8–10 mEq/L in 24 hours** to prevent **Osmotic Demyelination Syndrome (Central Pontine Myelinolysis)**.

Q: What is the normal urinary anion gap?

Zero. **Explanation:** The **Urinary Anion Gap (UAG)** is a clinical tool used to indirectly estimate the concentration of ammonium ($NH_4^+$) in the urine, which is difficult to measure directly. It is calculated using the formula: **UAG = $[Na^+] + [K^+] - [Cl^-]$** 1. **Why "Zero" is the correct answer:** In a healthy individual with normal renal function and acid-base balance, the sum of measured cations ($Na^+$ and $K^+$) is approximately equal to the measured anion ($Cl^-$). Therefore, the normal UAG is typically **zero or slightly positive** (ranging from 0 to +20 mEq/L). This reflects a baseline state where the kidneys are not required to excrete excess acid. 2. **Why other options are incorrect:** * **Negative:** A negative UAG (e.g., -20 to -50 mEq/L) occurs when there is an increase in unmeasured cations, primarily $NH_4^+$. This is the expected physiological response in **diarrhea** (Hyperchloremic Metabolic Acidosis with intact renal acidification). * **Positive:** While a "normal" gap can be slightly positive, a significantly positive UAG in the presence of metabolic acidosis indicates a failure of the kidneys to excrete $NH_4^+$. This is characteristic of **Distal Renal Tubular Acidosis (Type 1 RTA)**. * **Cannot be determined:** UAG is easily determined using a spot urine sample for electrolytes. **NEET-PG High-Yield Pearls:** * **Mnemonic for Negative UAG:** **"NeGUTive"** – A negative UAG points toward a **GUT** (gastrointestinal) cause of acidosis, like diarrhea. * **Positive UAG in Acidosis:** Suggests **RTA** (Renal cause). * The UAG is only valid for diagnosing **Normal Anion Gap Metabolic Acidosis (NAGMA)**. It is not used for High Anion Gap Metabolic Acidosis (HAGMA).

Q: Which of the following defines hyperkalemia?

Serum level >5.5 mEq/L. **Explanation:** **1. Why Option A is Correct:** Hyperkalemia is defined as a serum potassium concentration greater than the upper limit of the normal range, which is typically **3.5 to 5.5 mEq/L**. Therefore, a level **>5.5 mEq/L** is the standard biochemical definition. Potassium is the primary intracellular cation; even minor shifts from the intracellular to extracellular compartment can lead to life-threatening cardiac arrhythmias. **2. Why the Other Options are Incorrect:** * **Option B (>6.5 mEq/L):** This represents **severe hyperkalemia**, which is a medical emergency requiring immediate intervention (e.g., calcium gluconate), but it is not the threshold for the initial diagnosis. * **Option C (T wave inversion):** This is typically seen in **hypokalemia** or myocardial ischemia. In hyperkalemia, the classic early ECG finding is **Tall, Tented (Peaked) T waves**. * **Option D (Peaking of P wave):** This is a sign of right atrial enlargement (*P-pulmonale*). In hyperkalemia, P waves actually become **flattened** or may disappear entirely as the potassium level rises. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **ECG Progression in Hyperkalemia:** Tall tented T waves → Prolonged PR interval → Loss of P wave → Widening of QRS complex → **Sine wave pattern** (pre-terminal). * **Pseudohyperkalemia:** A common "distractor" in exams; it occurs due to in-vitro hemolysis during blood collection or in cases of extreme thrombocytosis/leukocytosis. * **Management Mnemonic (C BIG K):** **C**alcium gluconate (stabilizes membrane), **B**icarbonate/Beta-agonists, **I**nsulin + **G**lucose (shifts K+ intracellularly), **K**ayexalate/Diuretics/Dialysis (removes K+). * **Aldosterone Connection:** Hyperkalemia is a potent stimulator of aldosterone secretion from the adrenal cortex to promote renal potassium excretion.

Question 1

Which is the most effective buffer system in the blood that is controlled by respiration?

Accepted Answer

Bicarbonates

Answer

Hemoglobin

Answer

Proteins

Answer

Phosphates

Question 2

High anion gap is seen in which of the following conditions?

Accepted Answer

Lactic acidosis

Answer

Renal failure

Answer

Diarrhea

Answer

Alcoholism

Question 3

All of the following are causes of hypovolemic hyponatremia with urine sodium > 20 mEq/L, EXCEPT?

Accepted Answer

Vomiting

Answer

Renal losses

Answer

Cerebral salt wasting syndrome

Answer

Mineralocorticoid deficiency

Question 4

What is the normal urinary anion gap?

Accepted Answer

Zero

Answer

Positive

Answer

Negative

Answer

Cannot be determined

Question 5

Which of the following defines hyperkalemia?

Accepted Answer

Serum level >5.5 mEq/L

Answer

Serum level >6.5 mEq/L

Answer

T wave inversion

Answer

Peaking of P wave

Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance — MCQs

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