Acid-Base 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: Which of the following can cause metabolic alkalosis?

Addison's disease. **Explanation:** **Correct Answer: B. Addison’s Disease** In Addison’s disease (primary adrenocortical insufficiency), there is a deficiency of **aldosterone**. Aldosterone normally acts on the distal tubules to reabsorb Na⁺ and secrete H⁺ and K⁺. Its absence leads to decreased H⁺ secretion, resulting in **Metabolic Acidosis** (specifically Normal Anion Gap Metabolic Acidosis). *Note: There appears to be a discrepancy in the provided key. Addison’s disease causes acidosis, while the other options (A and C) are classic causes of alkalosis. In the context of standard physiology:* * **Option A (Furosemide):** Causes **Metabolic Alkalosis**. Loop diuretics inhibit the Na-K-2Cl cotransporter, increasing distal delivery of Na⁺, which promotes H⁺ and K⁺ secretion (Contraction alkalosis). * **Option C (Hypokalemia):** Causes **Metabolic Alkalosis**. When extracellular K⁺ is low, K⁺ shifts out of cells in exchange for H⁺ shifting into cells, raising extracellular pH. It also promotes "paradoxical aciduria." * **Option D (Hyponatremia):** This is an electrolyte imbalance, not a primary cause of acid-base disturbance, though it often co-exists with various states. **High-Yield NEET-PG Pearls:** 1. **Conn’s Syndrome (Hyperaldosteronism):** The opposite of Addison’s; it causes **Metabolic Alkalosis** and Hypokalemia. 2. **Vomiting/NG Suction:** The most common clinical cause of metabolic alkalosis due to loss of HCl. 3. **Saline Responsive vs. Resistant:** Alkalosis due to diuretics or vomiting is "Saline Responsive" (Urinary Cl⁻ 20 mEq/L).

Q: Metabolic alkalosis is seen in all the following conditions except?

Methanol poisoning. **Explanation:** The correct answer is **Methanol poisoning** because it causes **High Anion Gap Metabolic Acidosis (HAGMA)**, not alkalosis. 1. **Methanol Poisoning (Correct Answer):** Methanol is metabolized by alcohol dehydrogenase into **formic acid**. The accumulation of formate ions and the associated increase in hydrogen ion concentration lead to a severe metabolic acidosis. This is often associated with an increased "osmolal gap." 2. **Why the other options are incorrect (Causes of Metabolic Alkalosis):** * **Vomiting:** Gastric juice is rich in HCl. Loss of stomach acid leads to a relative increase in bicarbonate levels (H+ loss). Additionally, volume depletion activates the Renin-Angiotensin-Aldosterone System (RAAS), further promoting bicarbonate reabsorption. * **Cushing’s Disease:** Excess cortisol has mineralocorticoid activity. This leads to increased secretion of H+ and K+ in the distal renal tubules, resulting in **hypokalemic metabolic alkalosis**. * **Diuretic Therapy:** Loop and Thiazide diuretics cause the loss of Na+, Cl-, and water. This leads to "contraction alkalosis" and increased distal delivery of sodium, which stimulates H+ secretion. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** "MUDPILES" (Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Conn’s Syndrome & Cushing’s:** Both are classic endocrine causes of metabolic alkalosis due to mineralocorticoid excess. * **Bartter’s and Gitelman’s Syndromes:** These are important genetic causes of metabolic alkalosis that mimic diuretic use (Loop and Thiazide-like effects, respectively).

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 is not seen in hyperventilation?

Hypocalcemia and hyperphosphatemia. **Explanation:** Hyperventilation leads to the excessive "blowing off" of $CO_2$, resulting in **Respiratory Alkalosis** (Option B). This physiological state triggers specific electrolyte shifts and neurological symptoms. **1. Why Option D is correct (The "Not Seen" finding):** In respiratory alkalosis, hydrogen ions ($H^+$) dissociate from albumin to buffer the high pH. This leaves more binding sites available for calcium, leading to a decrease in ionized (active) calcium—**Hypocalcemia**. Simultaneously, alkalosis stimulates intracellular glycolysis, which consumes inorganic phosphate to produce phosphorylated glycolytic intermediates. This causes phosphate to shift from the extracellular to the intracellular compartment, resulting in **Hypophosphatemia**. Therefore, **Hyperphosphatemia** is not seen; instead, both calcium and phosphate levels decrease in the serum. **2. Why other options are incorrect:** * **Option A (Seizures):** Alkalosis increases neuronal excitability. Furthermore, the decrease in $PaCO_2$ causes cerebral vasoconstriction, reducing cerebral blood flow, which can trigger seizures. * **Option B (Alkalosis):** Hyperventilation directly reduces $H_2CO_3$ levels, raising the blood pH (Respiratory Alkalosis). * **Option C (Hypocalcemia and Hypophosphatemia):** As explained above, these are the classic electrolyte disturbances associated with acute respiratory alkalosis. **Clinical Pearls for NEET-PG:** * **Chvostek’s and Trousseau’s signs:** These may be positive during hyperventilation due to functional hypocalcemia (low ionized $Ca^{2+}$), even if total serum calcium is normal. * **Carpopedal Spasm:** A hallmark clinical sign of hyperventilation-induced hypocalcemia. * **Cerebral Blood Flow:** $CO_2$ is a potent vasodilator. Low $CO_2$ (hypocapnia) causes vasoconstriction, which is why hyperventilation is used therapeutically to acutely lower intracranial pressure (ICP).

Q: In a person with normal kidneys and normal lungs who has chronic metabolic acidosis, what would be expected compared to normal, except:

Increased plasma HCO3- concentration. **Explanation:** In **chronic metabolic acidosis**, the primary pathology is a decrease in plasma bicarbonate ($HCO_3^-$) concentration, either due to increased acid production or excessive loss of base. Therefore, **Option D** is the correct answer because it describes the exact opposite of the clinical state; plasma $HCO_3^-$ will be **decreased**, not increased. **Why the other options are incorrect (Expected findings in Acidosis):** * **Option A (Increased NH4Cl excretion):** To compensate for acidosis, the kidneys increase the synthesis of ammonia ($NH_3$) from glutamine. $NH_3$ buffers $H^+$ to form ammonium ($NH_4^+$), which is excreted as $NH_4Cl$. This is the most important mechanism for the excretion of "fixed" acids. * **Option B (Decreased urine pH):** The kidneys attempt to eliminate excess $H^+$ ions via $H^+$-ATPase pumps in the intercalated cells of the collecting duct. This increases the acidity of the urine, lowering its pH (minimum possible pH is ~4.5). * **Option C (Decreased urine $HCO_3^-$ excretion):** In acidosis, the filtered load of $HCO_3^-$ is low, and the kidneys ensure that virtually 100% of the filtered $HCO_3^-$ is reabsorbed to preserve the body's buffering capacity. **High-Yield Clinical Pearls for NEET-PG:** * **Winter’s Formula:** Used to calculate expected $PCO_2$ compensation in metabolic acidosis: $PCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$. * **Anion Gap:** Always calculate the Anion Gap ($Na^+ - [Cl^- + HCO_3^-]$) to differentiate causes (e.g., MUDPILES for High Anion Gap vs. Diarrhea/RTA for Normal Anion Gap). * **Rate-limiting step:** Glutamine metabolism in the proximal tubule is the primary source of new $HCO_3^-$ and $NH_4^+$ production during chronic acidosis.

Q: A person presents with the following parameters: PCO3- 30 mm of Hg, PO2- 105 mm of Hg, pH-7.45. What is the underlying acid-base disorder?

Respiratory alkalosis. ### Explanation To solve acid-base problems, follow a systematic three-step approach: **1. Analyze the pH:** The normal range for arterial pH is **7.35–7.45**. Here, the pH is **7.45**. While this is at the upper limit of normal, it indicates a trend toward **alkalosis**. **2. Analyze the Respiratory Component ($PCO_2$):** The normal $PCO_2$ is **40 mmHg** (range 35–45). In this case, $PCO_2$ is **30 mmHg**. A low $PCO_2$ (hypocapnia) signifies that CO₂ (an acid) is being "washed out," which leads to **Respiratory Alkalosis**. **3. Determine the Primary Disorder:** Since the low $PCO_2$ (alkalotic change) matches the pH trend (alkalosis), the primary disorder is **Respiratory Alkalosis**. The pH of 7.45 suggests this is likely a **compensated** state (where the kidneys have excreted $HCO_3^-$ to bring the pH back to the normal range). --- ### Why the other options are incorrect: * **Metabolic Acidosis:** Would present with a low pH ( 7.45) and high $HCO_3^-$. * **Respiratory Acidosis:** Would present with a low pH ( 45 mmHg) due to CO₂ retention. --- ### NEET-PG High-Yield Pearls: * **Rule of Thumb:** If the pH and $PCO_2$ move in **opposite** directions, the primary problem is **Respiratory**. If they move in the **same** direction, it is **Metabolic** (ROME: Respiratory Opposite, Metabolic Equal). * **Common Causes of Respiratory Alkalosis:** Hyperventilation (anxiety), high altitude (hypoxia-induced), pulmonary embolism, and early salicylate poisoning. * **Compensation:** In acute respiratory alkalosis, for every 10 mmHg drop in $PCO_2$, $HCO_3^-$ drops by 2 mEq/L. In chronic cases, it drops by 4–5 mEq/L.

Q: What is the typical effect of metabolic alkalosis on serum potassium levels?

Low potassium. **Explanation:** **Metabolic alkalosis** is characterized by a primary increase in serum bicarbonate ($HCO_3^-$) and an increase in blood pH. The resulting **hypokalemia** (low potassium) occurs due to two primary mechanisms: 1. **Intracellular Shift:** To compensate for the high extracellular pH, hydrogen ions ($H^+$) move out of the cells into the ECF. To maintain electroneutrality, potassium ions ($K^+$) shift from the ECF into the cells, lowering serum levels. 2. **Renal Excretion:** In alkalosis, the distal tubule attempts to conserve $H^+$ ions. This is achieved via the $H^+/K^+$ exchange pump; as the kidney retains $H^+$, it must excrete $K^+$ into the urine. **Analysis of Incorrect Options:** * **Option B (High calcium):** Alkalosis actually leads to **hypocalcemia** symptoms. High pH causes more calcium to bind to albumin, reducing the "ionized" (active) fraction of calcium, which can trigger tetany. * **Options C & D (Iodine trapping):** These options relate to thyroid physiology and the Sodium-Iodide Symporter (NIS). Iodine trapping is primarily regulated by TSH levels and is not directly influenced by acute changes in acid-base balance. **NEET-PG High-Yield Pearls:** * **The "Rule of Reciprocity":** Alkalosis leads to hypokalemia; Acidosis leads to hyperkalemia (except in cases of organic acid accumulation like lactic acidosis). * **Vomiting:** A classic cause of metabolic alkalosis that results in "Paradoxical Aciduria"—the body prioritizes sodium/volume conservation over pH, leading to $H^+$ excretion despite systemic alkalosis. * **Correction:** In metabolic alkalosis, you must often correct the potassium deficit before the pH can normalize.

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

Metabolic alkalosis is seen in:

Accepted Answer

Primary mineralocorticoid excess

Answer

Deficiency of mineralocorticoid

Answer

Decreased acid excretion

Answer

Decreased base excretion

Question 3

Which of the following can cause metabolic alkalosis?

Accepted Answer

Addison's disease

Answer

Furosemide

Answer

Hypokalemia

Answer

Hyponatremia

Question 4

Metabolic alkalosis is seen in all the following conditions except?

Accepted Answer

Methanol poisoning

Answer

Cushing's disease

Answer

Vomiting

Answer

Diuretic therapy

Question 5

What is the normal urinary anion gap?

Accepted Answer

Zero

Answer

Positive

Answer

Negative

Answer

Cannot be determined

Question 6

Which of the following is not seen in hyperventilation?

Accepted Answer

Hypocalcemia and hyperphosphatemia

Answer

Seizures

Answer

Alkalosis

Answer

Hypocalcemia and hypophosphatemia

Question 7

In a person with normal kidneys and normal lungs who has chronic metabolic acidosis, what would be expected compared to normal, except:

Accepted Answer

Increased plasma HCO3- concentration

Answer

Increased renal excretion of NH4Cl

Answer

Decreased urine pH

Answer

Decreased urine HCO3- excretion

Question 8

A person presents with the following parameters: PCO3- 30 mm of Hg, PO2- 105 mm of Hg, pH-7.45. What is the underlying acid-base disorder?

Accepted Answer

Respiratory alkalosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Question 9

What is the typical effect of metabolic alkalosis on serum potassium levels?

Accepted Answer

Low potassium

Answer

High calcium

Answer

Low iodine trapping by follicular cells

Answer

High iodine trapping by follicular cells

Question 10

Which of the following is true about the Anion Gap?

Accepted Answer

It is the difference between unmeasured anions and unmeasured cations.

Answer

The normal value is 20 mEq/L.

Answer

The anion gap is increased in diarrhea.

Answer

The anion gap increases if unmeasured cations increase.

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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