Acid-Base and Electrolyte Balance Practice Questions

Q: A normal-anion-gap metabolic acidosis occurs in patients with?

Diarrhoea. **Explanation:** Metabolic acidosis is categorized based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. A **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as hyperchloremic acidosis, occurs when the loss of bicarbonate ($HCO_3^-$) is replaced by a proportional increase in chloride ($Cl^-$) to maintain electroneutrality. **1. Why Diarrhoea is correct:** Lower gastrointestinal secretions are rich in bicarbonate. In **diarrhoea**, there is a direct loss of $HCO_3^-$ from the body. To compensate for the loss of negative charges, the kidneys retain chloride, leading to a normal anion gap but elevated chloride levels (Hyperchloremic NAGMA). **2. Why the other options are incorrect:** * **Diabetic Ketoacidosis (DKA):** Characterized by the accumulation of unmeasured anions (acetoacetate and beta-hydroxybutyrate), leading to a **High Anion Gap Metabolic Acidosis (HAGMA)**. * **Methyl Alcohol Poisoning:** Metabolism of methanol produces formic acid. These exogenous acid anions increase the anion gap (**HAGMA**). * **Acute Renal Failure:** Failure to excrete fixed acids (phosphates, sulfates) results in an accumulation of unmeasured anions, causing **HAGMA**. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for NAGMA (USED CARP):** **U**reterosigmoidostomy, **S**aline infusion, **E**ndocrine (Addison’s), **D**iarrhoea, **C**arbonic anhydrase inhibitors (Acetazolamide), **A**mmonium chloride, **R**enal tubular acidosis (RTA), **P**ancreatic fistula. * **Mnemonic for HAGMA (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 **RTA or Diarrhoea**, always think **NAGMA**. If it mentions **toxins or renal failure**, think **HAGMA**.

Q: In metabolic acidosis caused by diabetic ketoacidosis, which of the following would be greater than normal?

Anion gap. In metabolic acidosis, specifically Diabetic Ketoacidosis (DKA), the primary pathology is the accumulation of fixed organic acids (acetoacetate and β-hydroxybutyrate). **1. Why the Anion Gap is increased:** The Anion Gap (AG) is calculated as: $[Na^+] - ([Cl^-] + [HCO_3^-])$. In DKA, the ketoacids dissociate into hydrogen ions ($H^+$) and unmeasured anions (ketoanions). The $H^+$ ions are buffered by $HCO_3^-$, causing the bicarbonate level to drop. However, the chloride level remains relatively stable. Because the $HCO_3^-$ is replaced by unmeasured ketoanions rather than chloride, the gap between measured cations and measured anions increases. This makes DKA a classic example of **High Anion Gap Metabolic Acidosis (HAGMA).** **2. Why other options are incorrect:** * **Option A (Plasma $HCO_3^-$):** In any metabolic acidosis, bicarbonate is consumed as it acts as the primary buffer for excess $H^+$ ions. Therefore, $HCO_3^-$ levels will be **lower** than normal. * **Option C (Arterial $pCO_2$):** The body attempts to compensate for metabolic acidosis through the respiratory system. The low pH stimulates chemoreceptors to increase the rate and depth of breathing (**Kussmaul respiration**), which "blows off" $CO_2$. Consequently, $pCO_2$ will be **lower** than normal (respiratory compensation). **Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Winter’s Formula:** Used to calculate expected $pCO_2$ compensation: $Expected\ pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$. * **Normal Anion Gap:** 8–12 mEq/L. DKA typically presents with an AG > 12.

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: The acid-base status of a patient reveals a pH=7.46 and pCO2=30 mm Hg. The patient has a partially compensated primary-

Respiratory alkalosis. ### Explanation To solve acid-base questions, follow a systematic 3-step approach: **1. Analyze the pH:** The normal pH range is 7.35–7.45. Here, **pH = 7.46**, which is >7.45, indicating **Alkalemia** (Alkalosis). **2. Identify the Primary Cause:** * **Respiratory:** Look at $pCO_2$ (Normal: 35–45 mmHg). A low $pCO_2$ (<35) causes alkalosis. * **Metabolic:** Look at $HCO_3^-$ (Normal: 22–26 mEq/L). A high $HCO_3^-$ causes alkalosis. In this case, **$pCO_2$ is 30 mmHg** (Low). Since low $CO_2$ (an acid) leads to an increase in pH, the primary disturbance is **Respiratory Alkalosis**. **3. Determine Compensation:** The pH is 7.46, which is outside the normal range but moving toward it. This indicates **partial compensation**. In primary respiratory alkalosis, the kidneys compensate by excreting $HCO_3^-$ to lower the pH back toward normal. --- ### Why the other options are incorrect: * **Metabolic Acidosis:** Would present with a low pH ( 7.45), the primary driver would be a high $HCO_3^-$, and $pCO_2$ would typically be high (compensatory hypoventilation). * **Respiratory Acidosis:** Would present with a low pH ( 45 mmHg). --- ### High-Yield Clinical Pearls for NEET-PG: * **ROME Mnemonic:** **R**espiratory **O**pposite (pH ↑, $pCO_2$ ↓ or vice versa), **M**etabolic **E**qual (pH ↑, $HCO_3^-$ ↑ or vice versa). * **Common Causes of Respiratory Alkalosis:** Hyperventilation (Anxiety), Hysteria, High altitude, Salicylate poisoning (early stage), and Pulmonary embolism. * **Compensation Rule:** The body *never* over-compensates. If the pH is >7.40, the primary process must be alkalosis.

Q: Hypocalcemia is characterized by all EXCEPT:

Shortening of Q-T interval in ECG. **Explanation:** The correct answer is **C (Shortening of Q-T interval in ECG)** because hypocalcemia actually causes **prolongation of the QT interval**. **1. Why Option C is the correct answer (The Exception):** In hypocalcemia, the plateau phase (Phase 2) of the cardiac action potential is lengthened because the movement of calcium through L-type channels is slowed. This delay in repolarization manifests on an ECG as a **prolonged ST segment** and, consequently, a **prolonged QT interval**. Conversely, *hypercalcemia* is what causes a shortened QT interval. **2. Why the other options are incorrect (Features of Hypocalcemia):** * **Option A & D:** Low extracellular calcium lowers the threshold for depolarization in excitable tissues (nerves and muscles), leading to neuromuscular irritability. This presents as **paresthesia** (numbness/tingling in the circumoral region and fingertips) and **carpopedal spasms** (Trousseau’s sign). * **Option B:** Increased neuronal excitability leads to **hyperreflexia** and the **Chvostek’s sign** (twitching of facial muscles upon tapping the facial nerve). **High-Yield Clinical Pearls for NEET-PG:** * **Trousseau’s Sign:** Carpal spasm induced by inflating a BP cuff above systolic pressure for 3 minutes; it is more sensitive and specific than Chvostek’s sign. * **Etiology:** Most common cause is hypoparathyroidism (often post-surgical) or Vitamin D deficiency. * **Correction:** Always check **Serum Albumin** levels. For every 1 g/dL drop in albumin below 4 g/dL, add 0.8 mg/dL to the measured calcium level to get the "Corrected Calcium."

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: $2 \times [Na^+] + Glucose + Urea$.

Question 1

Type B lactic acidosis is due to what condition?

Accepted Answer

Diabetes

Answer

Congestive heart failure

Answer

Short bowel syndrome

Answer

Cyanide poisoning

Question 2

A normal-anion-gap metabolic acidosis occurs in patients with?

Accepted Answer

Diarrhoea

Answer

Diabetic ketoacidosis

Answer

Methyl alcohol poisoning

Answer

Acute renal failure

Question 3

In metabolic acidosis caused by diabetic ketoacidosis, which of the following would be greater than normal?

Accepted Answer

Anion gap

Answer

Concentration of plasma HCO3-

Answer

Arterial pCO2

Answer

All of the above

Question 4

20 mEq (mmol) of potassium chloride in 500 ml of 5% dextrose solution is given intravenously to treat which of the following?

Accepted Answer

Metabolic alkalosis

Answer

Respiratory alkalosis

Answer

Metabolic acidosis

Answer

Respiratory acidosis

Question 5

Hypomagnesemia presents with all of the following except:

Accepted Answer

Association with diabetic ketoacidosis

Answer

Symptoms similar to hypocalcemia

Answer

Development of torsades de pointes

Answer

Potentiation of hypocalcemia

Question 6

Which of the following causes hyperchloremic metabolic acidosis?

Accepted Answer

Diarrhea

Answer

Ethylene glycol poisoning

Answer

Diabetic ketoacidosis (DKA)

Answer

Lactic acidosis

Question 7

Increased anion gap is a characteristic feature of which of the following conditions?

Accepted Answer

Hyperosmolar non-ketotic diabetic coma

Answer

Hypoglycemic coma

Answer

Phenformin toxicity with coma

Answer

Renal tubular acidosis

Question 8

The acid-base status of a patient reveals a pH=7.46 and pCO2=30 mm Hg. The patient has a partially compensated primary-

Accepted Answer

Respiratory alkalosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Question 9

Hypocalcemia is characterized by all EXCEPT:

Accepted Answer

Shortening of Q-T interval in ECG

Answer

Numbness and tingling of the circumoral region

Answer

Hyperactive tendon reflexes and positive Chvostek's sign

Answer

Carpopedal spasm

Question 10

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

Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance — MCQs

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