Acid-Base Balance Practice Questions

Q: Sine wave pattern on ECG is seen when serum potassium exceeds which of the following values in mEq/dl?

> 8 mEq/dl. **Explanation:** The **sine wave pattern** is a critical, life-threatening ECG finding indicating severe hyperkalemia. It occurs due to the progressive slowing of cardiac conduction and the fusion of the widened QRS complex with the T-wave. **1. Why Option C is Correct:** As serum potassium levels rise above **8.0–9.0 mEq/L**, the resting membrane potential of myocytes becomes significantly depolarized (less negative). This leads to the inactivation of sodium channels, causing a marked decrease in conduction velocity. The P-wave eventually disappears (atrial standstill), and the QRS complex widens severely until it merges with the peaked T-wave, forming a rhythmic, undulating "sine wave." This is a pre-terminal rhythm that often precedes ventricular fibrillation or asystole. **2. Why Other Options are Incorrect:** * **Option A (> 6 mEq/L):** At this level (mild to moderate hyperkalemia), the earliest ECG change is typically **tall, peaked "tented" T-waves**, best seen in precordial leads. * **Option B (> 7 mEq/L):** At this stage, we observe the **loss of P-waves** (atrial paralysis) and the beginning of **QRS widening**. * **Option D (> 10 mEq/L):** While a sine wave is certainly present at this level, it typically manifests earlier (around 8–9 mEq/L). Waiting for 10 mEq/L would be clinically fatal, as cardiac arrest usually occurs before this threshold. **High-Yield Clinical Pearls for NEET-PG:** * **Sequence of ECG changes:** Peaked T-waves → PR prolongation → Loss of P-wave → QRS widening → Sine wave → Ventricular Fibrillation. * **Treatment Priority:** The first step in managing hyperkalemia with ECG changes is **Intravenous Calcium Gluconate** (to stabilize the cardiac membrane), followed by insulin/dextrose to shift potassium intracellularly. * **Pseudohyperkalemia:** Always rule out hemolysis during blood collection if ECG is normal despite high lab values.

Q: A patient's acid-base status reveals a pH of 7.46 and pCO2 of 30 mm Hg. What is the underlying acid-base disorder?

Respiratory alkalosis. ### Explanation To solve any acid-base question, follow a systematic three-step approach: **1. Determine the pH status:** The normal arterial pH range is **7.35 – 7.45**. In this patient, the pH is **7.46**, which is >7.45. This indicates **Alkalemia** (Alkalosis). **2. Identify the primary cause (Respiratory vs. Metabolic):** * **Respiratory:** Look at $pCO_2$ (Normal: 35–45 mmHg). $CO_2$ acts as an acid. A decrease in $pCO_2$ (hypocapnia) causes alkalosis. * **Metabolic:** Look at $HCO_3^-$ (Normal: 22–26 mEq/L). In this case, the $pCO_2$ is **30 mmHg** (Low). Since a low $pCO_2$ matches the alkaline pH, the primary disorder is **Respiratory Alkalosis**. --- #### Why the other options are incorrect: * **Metabolic Acidosis:** Would present with a low pH ( 7.45) and a high $HCO_3^-$. * **Respiratory Acidosis:** Would present with a low pH ( 45 mmHg) due to hypoventilation. --- #### High-Yield Clinical Pearls for NEET-PG: * **Common Causes:** Hyperventilation (anxiety, pain), high altitude (hypoxia-induced), pulmonary embolism, and early salicylate poisoning. * **Compensation:** In acute respiratory alkalosis, for every 10 mmHg drop in $pCO_2$, the $HCO_3^-$ drops by **2 mEq/L**. In chronic cases, it drops by **4–5 mEq/L**. * **Electrolyte Shift:** Alkalosis leads to a decrease in ionized calcium (as calcium binds more to albumin), which can cause **tetany** or paresthesia despite normal total serum calcium levels.

Q: Interpret the following lab values: pH 7.42, PaCO2 25 mmHg, HCO3- 18 mEq/L.

Respiratory alkalosis, fully compensated. ### Explanation To interpret acid-base disorders, follow a systematic three-step approach: 1. **Determine the pH status:** The pH is **7.42**. Since it falls within the normal range (7.35–7.45) but is on the alkaline side of the midpoint (7.40), it indicates a **fully compensated alkalosis**. 2. **Identify the primary cause:** * The **PaCO2 is 25 mmHg** (Normal: 40 mmHg). Low CO2 (hypocapnia) causes alkalosis. * The **HCO3- is 18 mEq/L** (Normal: 24 mEq/L). Low bicarbonate causes acidosis. Since the low PaCO2 matches the alkaline pH trend, the primary disturbance is **Respiratory Alkalosis**. 3. **Assess Compensation:** The kidneys have excreted HCO3- to bring the pH back into the normal range. Because the pH is now normal, it is **fully compensated**. #### Why the other options are incorrect: * **A & B (Partially compensated):** In partial compensation, the pH remains abnormal (below 7.35 or above 7.45). Here, the pH is 7.42 (normal). * **C (Respiratory acidosis):** In respiratory acidosis, the PaCO2 would be elevated (>45 mmHg) and the pH would be on the acidic side of normal ( 7.40 is compensated alkalosis. * **Renal Compensation Speed:** Metabolic compensation for respiratory disorders is slow (takes 2–5 days), whereas respiratory compensation for metabolic disorders is rapid (minutes to hours). * **Common Cause:** Hyperventilation (due to high altitude, anxiety, or pulmonary embolism) is the most frequent cause of respiratory alkalosis.

Q: A patient presents with a pH of 7.2, PO2 of 50 mm Hg, and HCO3– of 13 mEq/L. These values are most consistent with which of the following acid-base disturbances?

Mixed respiratory and metabolic acidosis. ### Explanation To solve acid-base problems, follow a systematic approach: **1. Analyze the pH:** The patient’s pH is **7.2** (Normal: 7.35–7.45). This indicates **acidemia**. **2. Identify the Metabolic Component (HCO3–):** The HCO3– is **13 mEq/L** (Normal: 22–26 mEq/L). A low bicarbonate level in the presence of acidemia signifies **metabolic acidosis**. **3. Identify the Respiratory Component (PCO2/PO2):** While PCO2 is the primary indicator for respiratory balance, we can infer the status from the clinical picture. In pure metabolic acidosis, the body should compensate by hyperventilating to "blow off" CO2 (Winter’s Formula). However, this patient has a **PO2 of 50 mm Hg** (Normal: 80–100 mm Hg), indicating significant **hypoxemia/hypoventilation**. If the lungs were compensating correctly, PO2 would typically be normal or elevated due to hyperpnea. The presence of respiratory failure alongside low bicarbonate confirms a **Mixed Respiratory and Metabolic Acidosis**. #### Why Incorrect Options are Wrong: * **Respiratory Acidosis (A):** While the low PO2 suggests a respiratory issue, the low HCO3– (13) cannot be a compensation for respiratory acidosis (which would cause HCO3– to rise). * **Metabolic Alkalosis (B) & Respiratory Alkalosis (C):** Both are ruled out immediately because the pH (7.2) is acidic, not alkaline (>7.45). #### High-Yield Clinical Pearls for NEET-PG: * **Mixed Disorders:** Suspect a mixed disorder when the compensation is inadequate or opposite to what is expected. * **Winter’s Formula:** Expected $PCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2$. If the actual $PCO_2$ is higher than expected, a concurrent respiratory acidosis exists. * **Common Scenario:** Mixed acidosis is frequently seen in **cardiopulmonary arrest** (lactic acidosis + respiratory failure) or severe **septic shock**.

Q: What is the trans-tubular potassium gradient (TTKG) in hypokalemia?

<3-4. **Explanation:** The **Transtubular Potassium Gradient (TTKG)** is a clinical tool used to estimate the conservation or secretion of potassium by the cortical collecting duct (CCD). It reflects the activity of aldosterone and the responsiveness of the distal tubule to it. **1. Why Option A is Correct:** In a patient with **hypokalemia**, a normal physiological response by the kidneys is to conserve potassium. If the kidneys are functioning correctly and the cause of hypokalemia is extra-renal (e.g., diarrhea or poor intake), the TTKG should be **low ( 6-10):** These values represent a high gradient. If a patient is hypokalemic but has a TTKG **>7**, it indicates **renal potassium wasting**. This suggests that the kidneys are inappropriately secreting potassium despite low serum levels, often due to hyperaldosteronism, diuretics, or renal tubular acidosis (RTA). **Clinical Pearls for NEET-PG:** * **Formula:** $TTKG = \frac{[K^+ \text{ urine} / (U/P \text{ osmolality})]}{[K^+ \text{ plasma}]}$ * **Prerequisites:** For TTKG to be accurate, the urine must be concentrated (Urine Osmolality > Plasma Osmolality) and urine sodium should be >25 mEq/L to ensure adequate delivery to the distal tubule. * **Hyperkalemia Context:** In a patient with hyperkalemia, a TTKG **<7** suggests mineralocorticoid deficiency (e.g., Addison’s disease) or resistance (e.g., Spironolactone).

Q: Which of the following is FALSE regarding bicarbonate (HCO3-)?

A 7.5% solution of bicarbonate contains 2 nanomoles.. ### Explanation **Why Option C is False (The Correct Answer):** The statement regarding the concentration of a 7.5% Sodium Bicarbonate ($NaHCO_3$) solution is mathematically incorrect. In medical pharmacology, a **7.5% solution** means 7.5 grams of the solute in 100 mL of solution. * **Calculation:** 7.5 g/100 mL is equivalent to 75 g/L. * The molecular weight of $NaHCO_3$ is approximately 84 g/mol. * Therefore, 75 g/L ÷ 84 g/mol ≈ **0.89 mol/L (or 890 millimoles/L)**. The claim that it contains "2 nanomoles" is an extreme underestimate, as the concentration is actually in the molar/millimolar range. **Analysis of Other Options:** * **Option A:** This is **True**. The normal range for arterial plasma bicarbonate is **22–28 mEq/L** (average ~24-25 mEq/L). It is the most important extracellular buffer. * **Option B:** This is **True**. Intracellular bicarbonate concentration is significantly lower than extracellular levels, typically averaging around **10 mEq/L** due to the negative resting membrane potential of cells which tends to repel anions. * **Option C:** This is **True**. In the proximal convoluted tubule (PCT), **Carbonic Anhydrase (Type IV)** on the brush border facilitates reabsorption, while **Carbonic Anhydrase (Type II)** in the cytoplasm helps generate "new" bicarbonate, which is then transported into the blood. **High-Yield Clinical Pearls for NEET-PG:** * **Bicarbonate Reabsorption:** 85% occurs in the PCT. It cannot be reabsorbed directly; it must first be converted to $CO_2$ and $H_2O$ by Carbonic Anhydrase. * **Henderson-Hasselbalch Equation:** $pH = 6.1 + \log ([HCO_3^-] / 0.03 \times PCO_2)$. * **Anion Gap:** Calculated as $Na^+ - (Cl^- + HCO_3^-)$. Normal is 8–12 mEq/L. Bicarbonate levels drop in High Anion Gap Metabolic Acidosis (HAGMA) as it is consumed buffering organic acids.

Question 1

Sine wave pattern on ECG is seen when serum potassium exceeds which of the following values in mEq/dl?

Accepted Answer

> 8 mEq/dl

Answer

> 6 mEq/dl

Answer

> 7 mEq/dl

Answer

> 10 mEq/dl

Question 2

A patient has a pH of 7.2, PCO2 of 12, and PO2 of 55. What is the most likely underlying cause?

Accepted Answer

Metabolic acidosis and respiratory alkalosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Question 3

A patient's acid-base status reveals a pH of 7.46 and pCO2 of 30 mm Hg. What is the underlying acid-base disorder?

Accepted Answer

Respiratory alkalosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Question 4

During the 'fight or flight' reaction, which of the following is responsible for an increase in local blood flow?

Accepted Answer

Sympathetic system-mediated cholinergic release

Answer

Local hormones

Answer

Parasympathetic cholinergic activity

Answer

Endocrine factors only

Question 5

Interpret the following lab values: pH 7.42, PaCO2 25 mmHg, HCO3- 18 mEq/L.

Accepted Answer

Respiratory alkalosis, fully compensated

Answer

Metabolic acidosis, partially compensated

Answer

Respiratory acidosis, partially compensated

Answer

Respiratory acidosis, fully compensated

Question 6

A patient has the following arterial blood gas values: pH = 7.30, pCO2 = 38 mmHg, HCO3 = 18 mEq/L. What is the most likely diagnosis?

Accepted Answer

Metabolic acidosis with respiratory acidosis

Answer

Metabolic acidosis with compensatory respiratory alkalosis

Answer

Respiratory acidosis with compensatory metabolic alkalosis

Answer

Respiratory alkalosis with compensatory metabolic acidosis

Question 7

A patient presents with a pH of 7.2, PO2 of 50 mm Hg, and HCO3– of 13 mEq/L. These values are most consistent with which of the following acid-base disturbances?

Accepted Answer

Mixed respiratory and metabolic acidosis

Answer

Respiratory acidosis

Answer

Metabolic alkalosis

Answer

Respiratory alkalosis

Question 8

Metabolic alkalosis is associated with which of the following?

Accepted Answer

Acetazolamide

Answer

Fanconi's anemia

Answer

Hypocalcemia

Answer

Triamterene

Question 9

What is the trans-tubular potassium gradient (TTKG) in hypokalemia?

Accepted Answer

<3-4

Answer

>6-7

Answer

>9-10

Answer

>10-15

Question 10

Which of the following is FALSE regarding bicarbonate (HCO3-)?

Accepted Answer

A 7.5% solution of bicarbonate contains 2 nanomoles.

Answer

Extracellular concentration is approximately 25 mEq/L.

Answer

Intracellular concentration is approximately 10 mEq/L.

Answer

In the kidney, bicarbonate is reabsorbed and produced by carbonic anhydrase.

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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