Acid-Base Balance — MCQs

Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 81: Metabolic alkalosis is seen in all of the following conditions except?

A. Thiazide diuretic therapy
B. Prolonged vomiting
C. Ureterosigmoidostomy (Correct Answer)
D. Cushing's disease

Explanation: **Explanation:** The correct answer is **Ureterosigmoidostomy**, which characteristically causes **Normal Anion Gap Metabolic Acidosis (NAGMA)**, not alkalosis. ### 1. Why Ureterosigmoidostomy causes Acidosis In a ureterosigmoidostomy, the ureters are diverted into the sigmoid colon. The colonic mucosa is exposed to urine for prolonged periods, leading to: * **Chloride-Bicarbonate Exchange:** The intestinal epithelium reabsorbs chloride from the urine in exchange for secreting bicarbonate into the lumen. * **Ammonium Reabsorption:** The colon reabsorbs ammonium ($NH_4^+$) from the urine, which is metabolized in the liver to urea and hydrogen ions ($H^+$). The net loss of $HCO_3^-$ and retention of $Cl^-$ results in **hyperchloremic metabolic acidosis**. ### 2. Why the other options are incorrect (Causes of Metabolic Alkalosis) * **Thiazide Diuretics:** These inhibit the $Na^+/Cl^-$ symporter in the distal tubule. Increased sodium delivery to the collecting duct promotes $K^+$ and $H^+$ secretion (via aldosterone activation), leading to "contraction alkalosis." * **Prolonged Vomiting:** Gastric juice is rich in $HCl$. Loss of stomach acid directly removes $H^+$ ions from the body. Additionally, the loss of fluid leads to volume depletion and secondary hyperaldosteronism, which further worsens alkalosis. * **Cushing’s Disease:** Excess cortisol has mineralocorticoid activity. It stimulates the $H^+$-ATPase pump in the intercalated cells of the collecting duct, causing increased $H^+$ excretion and $HCO_3^-$ regeneration. ### High-Yield Clinical Pearls for NEET-PG * **Mnemonic for NAGMA:** "USED CARP" (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhea, Carbonic anhydrase inhibitors, Adrenal insufficiency, Renal tubular acidosis, Pancreatic fistula). * **Vomiting vs. Diarrhea:** Vomiting causes Alkalosis (loss of acid); Diarrhea causes Acidosis (loss of alkali). * **Conn’s and Cushing’s:** Both cause hypertension, hypokalemia, and metabolic alkalosis.

Question 82: What is the most important buffer system present in the distal convoluted tubule?

A. Bicarbonate
B. Phosphate
C. Protein
D. Ammonia (Correct Answer)

Explanation: ### Explanation The correct answer is **Ammonia (D)**. In the distal convoluted tubule (DCT) and collecting ducts, the kidneys must excrete metabolic acids ($H^+$) to maintain acid-base homeostasis. While the bicarbonate system is the primary buffer in the blood, it is largely reabsorbed in the proximal tubule. In the distal segments, the **Ammonia ($NH_3/NH_4^+$) buffer system** becomes the most important and quantitatively significant mechanism for excreting $H^+$. * **Mechanism:** $NH_3$ is synthesized from Glutamine in the tubular cells and diffuses into the lumen. It combines with secreted $H^+$ to form $NH_4^+$ (Ammonium). Because the tubular membrane is impermeable to $NH_4^+$, it becomes "trapped" in the lumen and is excreted in the urine. This is the only buffer system that can be **physiologically regulated** (upregulated) in response to chronic acidosis. #### Why other options are incorrect: * **Bicarbonate (A):** Most bicarbonate (80-90%) is reabsorbed in the proximal tubule. By the time the filtrate reaches the DCT, bicarbonate levels are too low to act as an effective buffer. * **Phosphate (B):** Known as the "Titratable Acid" buffer. While important, its concentration is fixed and limited by dietary intake. It cannot be increased significantly during severe acidosis, unlike Ammonia. * **Protein (C):** Proteins are the most important **intracellular** buffers. Under normal physiological conditions, the DCT filtrate is virtually protein-free. #### High-Yield Clinical Pearls for NEET-PG: * **Glutamine** is the primary precursor for ammonia production in the kidneys. * **Diffusion Trapping:** The process where $NH_3$ (lipid-soluble) diffuses into the lumen and turns into $NH_4^+$ (water-soluble), preventing its reabsorption. * In **Chronic Metabolic Acidosis**, the ammonia buffer system is the most important adaptive response, increasing its capacity manifold.

Question 83: ABG analysis of a patient on ventilator shows decreased pCO2, normal pO2, and a pH of 7.5. What is the diagnosis?

A. Respiratory acidosis
B. Metabolic alkalosis
C. Respiratory alkalosis (Correct Answer)
D. Metabolic acidosis

Explanation: ### Explanation To diagnose acid-base disturbances, follow a systematic three-step approach: 1. **Check the pH:** The normal range is 7.35–7.45. Here, the **pH is 7.5**, indicating **Alkalosis** (pH > 7.45). 2. **Check the pCO2:** The normal range is 35–45 mmHg. Here, the **pCO2 is decreased**. Since $CO_2$ acts as an acid in the body (via carbonic acid), a decrease in $CO_2$ leads to an increase in pH. 3. **Determine the Primary Cause:** Because the pH (alkalosis) and pCO2 (low) move in opposite directions, the primary disturbance is **Respiratory Alkalosis**. **Why the other options are incorrect:** * **Respiratory Acidosis:** Would present with a low pH (< 7.35) and an elevated pCO2 (> 45 mmHg). * **Metabolic Alkalosis:** While the pH would be high (> 7.45), the primary driver would be an increased $HCO_3^-$ (bicarbonate), and pCO2 would typically be normal or slightly elevated as compensation. * **Metabolic Acidosis:** Would present with a low pH (< 7.35) and a primary decrease in $HCO_3^-$. **Clinical Pearls for NEET-PG:** * **Ventilator-Induced Alkalosis:** In patients on mechanical ventilation, respiratory alkalosis is often caused by **iatrogenic hyperventilation** (excessive tidal volume or respiratory rate), which "washes out" $CO_2$. * **ROME Mnemonic:** **R**espiratory **O**pposite (pH and $CO_2$ move in opposite directions), **M**etabolic **E**qual (pH and $HCO_3^-$ move in the same direction). * **Acute vs. Chronic:** 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 due to renal compensation.

Question 84: A patient has a pCO2 of 80 mm Hg and plasma bicarbonate of 33 mEq/L. What is the acid-base disorder?

A. Metabolic acidosis
B. Respiratory acidosis (Correct Answer)
C. Respiratory alkalosis
D. Excessive renal bicarbonate loss

Explanation: **Explanation:** The primary step in analyzing acid-base disorders is identifying the deviation in **pCO2** (normal: 40 mmHg) and **Bicarbonate (HCO3-)** (normal: 24 mEq/L). 1. **Why Respiratory Acidosis is Correct:** The patient has a significantly elevated pCO2 (80 mmHg), which is much higher than the normal 40 mmHg. In physiology, CO2 acts as a volatile acid. An increase in pCO2 (hypercapnia) indicates alveolar hypoventilation, leading to **Respiratory Acidosis**. The elevated bicarbonate (33 mEq/L) represents a compensatory response by the kidneys to buffer the acidity, suggesting this may be a chronic or partially compensated state. 2. **Why Incorrect Options are Wrong:** * **Metabolic Acidosis:** This would be characterized by a *low* bicarbonate (<22 mEq/L) and a low pH. Here, bicarbonate is elevated. * **Respiratory Alkalosis:** This occurs when pCO2 is *low* (<35 mmHg) due to hyperventilation. This patient’s pCO2 is doubled. * **Excessive Renal Bicarbonate Loss:** This mechanism leads to metabolic acidosis (e.g., Renal Tubular Acidosis). In this case, the kidneys are actually *retaining* bicarbonate to compensate for the high CO2. **High-Yield Clinical Pearls for NEET-PG:** * **The 1-2-3-4 Rule for Compensation:** * **Acute Resp. Acidosis:** HCO3 increases by **1** for every 10 mmHg rise in pCO2. * **Chronic Resp. Acidosis:** HCO3 increases by **3.5 to 4** for every 10 mmHg rise in pCO2. * In this case, pCO2 rose by 40 units. If acute, HCO3 should be ~28; if chronic, ~40. At 33 mEq/L, this patient is in a state of partially compensated respiratory acidosis. * **Common Causes:** COPD, Opioid overdose, Guillain-Barré syndrome, and Obstructive Sleep Apnea.

Question 85: Which of the following conditions is associated with normal anion gap metabolic acidosis?

A. Lactic acidosis
B. Ketoacidosis
C. Methanol poisoning
D. Hyperchloremic acidosis (Correct Answer)

Explanation: **Explanation:** Metabolic acidosis is categorized based on the **Anion Gap (AG)**, calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. A normal anion gap is typically 8–12 mEq/L. **Why the correct answer is right:** **Hyperchloremic acidosis** is the hallmark of **Normal Anion Gap Metabolic Acidosis (NAGMA)**. In this condition, the loss of bicarbonate ($HCO_3^-$) from the body (via the GI tract or kidneys) is compensated by a proportional increase in serum chloride ($Cl^-$) to maintain electroneutrality. Since the sum of chloride and bicarbonate remains constant, the calculated anion gap does not change. Common causes include diarrhea and Renal Tubular Acidosis (RTA). **Why the incorrect options are wrong:** * **Lactic acidosis (A), Ketoacidosis (B), and Methanol poisoning (C)** are all causes of **High Anion Gap Metabolic Acidosis (HAGMA)**. * In these conditions, metabolic acidosis occurs due to the accumulation of "unmeasured" organic acids (lactate, acetoacetate, or formic acid). As these acids dissociate, the $H^+$ ions consume $HCO_3^-$, but the remaining acid anions are not chloride. This increases the gap between measured cations and anions. **High-Yield NEET-PG Pearls:** * **Mnemonic for NAGMA (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, **P**ancreatic fistula. * **Mnemonic for HAGMA (MUDPILES):** **M**ethanol, **U**remia, **D**KA, **P**araldehyde, **I**NH/Iron, **L**actic acidosis, **E**thylene glycol, **S**alicylates. * **Gold Standard:** Diarrhea is the most common cause of NAGMA worldwide.

Question 86: What is the important role of the distal tubule of the kidney in acid-base balance?

A. Secretion of Ammonia (Correct Answer)
B. Secretion of bicarbonate
C. Secretion of HCl
D. Absorption of Ammonia

Explanation: **Explanation:** The kidneys maintain acid-base homeostasis primarily through two mechanisms: the reabsorption of filtered bicarbonate ($HCO_3^-$) and the excretion of fixed acids. While most bicarbonate reabsorption occurs in the proximal tubule, the **distal tubule and collecting ducts** are the primary sites for **active net acid excretion**. **1. Why "Secretion of Ammonia" is Correct:** In the distal nephron (specifically the intercalated cells), $H^+$ ions are actively secreted into the tubular lumen. However, the minimum urinary pH is limited to about 4.5. To excrete more acid, $H^+$ must be buffered. **Ammonia ($NH_3$)**, produced from glutamine metabolism, diffuses into the distal tubule where it combines with secreted $H^+$ to form **Ammonium ($NH_4^+$)**. Because $NH_4^+$ is lipid-insoluble, it becomes "trapped" in the lumen and is excreted. This "Ammonia trapping" is the most important adaptive mechanism for excreting a large acid load. **2. Analysis of Incorrect Options:** * **B. Secretion of bicarbonate:** Under normal physiological conditions, the kidney aims to *conserve* bicarbonate, not secrete it. Bicarbonate secretion only occurs in specific alkalotic states via Type B intercalated cells. * **C. Secretion of HCl:** The kidney does not secrete hydrochloric acid directly; it secretes hydrogen ions which may pair with chloride, but "HCl secretion" is a gastric process. * **D. Absorption of Ammonia:** Ammonia is produced and secreted into the lumen to facilitate acid excretion; absorbing it would be counterproductive to acid-base balance. **Clinical Pearls for NEET-PG:** * **Glutamine** is the primary precursor for ammonia production in the proximal tubule. * **Aldosterone** stimulates $H^+$ secretion in the alpha-intercalated cells of the distal tubule. * **Renal Tubular Acidosis (Type 1/Distal):** Characterized by the inability of the distal tubule to secrete $H^+$, leading to a high urinary pH (>5.5) and systemic acidosis.

Question 87: What is the primary determinant of the anion gap?

A. Proteins (Correct Answer)
B. Sulphates
C. Phosphates
D. Nitrates

Explanation: **Explanation:** The **Anion Gap (AG)** represents the difference between measured cations (Sodium) and measured anions (Chloride and Bicarbonate). It reflects the concentration of **unmeasured anions** in the plasma. **Why Proteins are the correct answer:** The formula for the anion gap is: $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$. In a healthy individual, the normal range is 8–12 mEq/L. **Albumin**, a plasma protein, carries a significant negative charge at physiological pH. It accounts for approximately **75-80%** of the normal anion gap. Therefore, proteins are the primary determinant. In clinical practice, if a patient has hypoalbuminemia, the "normal" anion gap must be adjusted downward (decreased by ~2.5 mEq/L for every 1 g/dL drop in albumin). **Analysis of Incorrect Options:** * **B & C (Sulphates and Phosphates):** While these are indeed "unmeasured anions" that contribute to the anion gap, their concentrations in the blood are significantly lower than that of plasma proteins under normal physiological conditions. They only rise significantly in conditions like renal failure. * **D (Nitrates):** Nitrates are not present in the blood in concentrations high enough to significantly influence the anion gap calculation. **High-Yield Clinical Pearls for NEET-PG:** * **MUDPILES:** The classic mnemonic for High Anion Gap Metabolic Acidosis (HAGMA) includes Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, and Salicylates. * **Albumin Correction:** Adjusted $AG = \text{Observed } AG + 2.5 \times (4.5 - \text{Measured Albumin})$. * **Goldmark:** A newer mnemonic for HAGMA (Glycols, Oxoproline, L-Lactate, D-Lactate, Methanol, Aspirin, Renal failure, Ketoacidosis).

Question 88: Uncompensated metabolic acidosis shows what pattern of pH and bicarbonate levels?

A. Increased pH with increased HCO3-
B. Increased pH with decreased HCO3-
C. Decreased pH with increased HCO3-
D. Decreased pH with decreased HCO3- (Correct Answer)

Explanation: ### Explanation **1. Understanding the Correct Answer (Option D)** In acid-base physiology, the pH is determined by the ratio of bicarbonate ($\text{HCO}_3^-$) to partial pressure of carbon dioxide ($\text{pCO}_2$), as described by the **Henderson-Hasselbalch equation**. * **Metabolic Acidosis** is primarily characterized by a **decrease in serum bicarbonate** ($\text{HCO}_3^-$ < 22 mEq/L). This loss of base or gain of fixed acid leads to an increase in hydrogen ion concentration, resulting in a **decreased pH** (< 7.35). * The term **"Uncompensated"** signifies that the body has not yet initiated (or is unable to perform) respiratory compensation. Therefore, the $\text{pCO}_2$ remains within the normal range, and the pH remains significantly abnormal. **2. Analysis of Incorrect Options** * **Option A (Increased pH, Increased $\text{HCO}_3^-$):** This describes **Metabolic Alkalosis**. * **Option B (Increased pH, Decreased $\text{HCO}_3^-$):** This pattern is seen in **Respiratory Alkalosis** (where low $\text{pCO}_2$ raises pH, and the kidneys may later decrease $\text{HCO}_3^-$ as compensation). * **Option C (Decreased pH, Increased $\text{HCO}_3^-$):** This describes **Respiratory Acidosis** (where high $\text{pCO}_2$ lowers pH, and the kidneys may later increase $\text{HCO}_3^-$ as compensation). **3. NEET-PG High-Yield Pearls** * **The Golden Rule:** In primary metabolic disorders, pH and $\text{HCO}_3^-$ always move in the **same direction**. * **Compensation:** In metabolic acidosis, the lungs compensate by hyperventilating to "blow off" $\text{CO}_2$ (**Kussmaul breathing**). If $\text{pCO}_2$ is low, it is *compensated*; if $\text{pCO}_2$ is normal, it is *uncompensated*. * **Winters' Formula:** Used to calculate the expected $\text{pCO}_2$ for compensation: $\text{pCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2$. * **Anion Gap:** Always calculate the Anion Gap ($\text{Na}^+ - [\text{Cl}^- + \text{HCO}_3^-]$) in metabolic acidosis to narrow the differential diagnosis (e.g., MUDPILES).

Question 89: Why is tetany seen with hyperventilation?

A. Metabolic alkalosis
B. Respiratory alkalosis (Correct Answer)
C. Metabolic acidosis
D. Respiratory acidosis

Explanation: ### Explanation **Why Respiratory Alkalosis is the Correct Answer:** Hyperventilation causes excessive "washout" of carbon dioxide ($CO_2$) from the lungs. According to the Henderson-Hasselbalch equation, a decrease in partial pressure of arterial $CO_2$ ($PaCO_2$) leads to an increase in blood pH, resulting in **Respiratory Alkalosis**. The development of **tetany** in this state is due to changes in ionized calcium levels. In an alkalotic state, there is a decrease in hydrogen ions ($H^+$). This causes plasma proteins (primarily albumin) to release $H^+$ ions to buffer the pH. To maintain electrical neutrality, **calcium ions ($Ca^{2+}$) bind to the newly vacant sites on albumin**. This reduces the concentration of **ionized (free) calcium** in the plasma, even though total serum calcium remains normal. Low ionized calcium increases neuronal permeability to sodium ions, leading to progressive depolarization and hyperexcitability of peripheral nerves, manifesting as tetany (e.g., carpopedal spasm). **Analysis of Incorrect Options:** * **Metabolic Alkalosis (A):** While this also causes decreased ionized calcium and tetany, it is caused by primary bicarbonate gain or $H^+$ loss (e.g., vomiting), not by hyperventilation. * **Metabolic Acidosis (C) & Respiratory Acidosis (D):** Acidosis increases ionized calcium levels because $H^+$ ions compete with calcium for albumin binding sites. Therefore, acidosis is protective against tetany. **High-Yield Clinical Pearls for NEET-PG:** * **Trousseau’s Sign:** Induction of carpopedal spasm by inflating a BP cuff above systolic pressure (more sensitive than Chvostek's). * **Chvostek’s Sign:** Tapping the facial nerve leads to twitching of facial muscles. * **Management:** For hyperventilation-induced tetany, breathing into a paper bag helps by rebreathing $CO_2$, which corrects the alkalosis and restores ionized calcium levels.

Question 90: Irrespective of the organ innervated, which neurotransmitter is NOT produced by postsynaptic parasympathetic nerve terminals/receptors?

A. Acetylcholine
B. Histamine (Correct Answer)
C. Noradrenaline
D. Dopamine

Explanation: ### Explanation The autonomic nervous system (ANS) is divided into the sympathetic and parasympathetic divisions. The **parasympathetic nervous system** is primarily cholinergic, meaning its postganglionic (postsynaptic) neurons release **Acetylcholine (ACh)** to act on muscarinic receptors at the target organ. **Why Histamine is the Correct Answer:** Histamine is a biogenic amine primarily produced by mast cells, basophils, and histaminergic neurons in the hypothalamus (tuberomammillary nucleus). It is **not** a neurotransmitter produced or released by postsynaptic parasympathetic nerve terminals. While histamine can influence autonomic functions, it does not serve as a parasympathetic neurotransmitter. **Analysis of Incorrect Options:** * **Acetylcholine (ACh):** This is the primary neurotransmitter for *all* preganglionic autonomic fibers and *all* postganglionic parasympathetic fibers. * **Noradrenaline & Dopamine:** While these are classic sympathetic neurotransmitters, they are also produced as **non-adrenergic, non-cholinergic (NANC)** transmitters in certain parasympathetic pathways. For example, in the gastrointestinal tract and urogenital system, some parasympathetic terminals release catecholamines or dopamine to modulate local blood flow or motility. **High-Yield NEET-PG Pearls:** * **NANC Transmitters:** Many parasympathetic nerves release co-transmitters alongside ACh, such as **Nitric Oxide (NO)** and **Vasoactive Intestinal Peptide (VIP)** (e.g., for penile erection). * **Exception to the Rule:** Postganglionic *sympathetic* fibers to sweat glands are **cholinergic** (release ACh), not noradrenergic. * **Histamine Receptors:** Remember the "1-2-3" rule: $H_1$ (Allergy/Gq), $H_2$ (Gastric acid/Gs), $H_3$ (Presynaptic inhibition).

Practice by Chapter

Acid-Base Chemistry

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Respiratory Regulation of Acid-Base Balance

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Renal Regulation of Acid-Base Balance

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Bicarbonate Buffer System

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Non-Bicarbonate Buffer Systems

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Respiratory Acidosis and Alkalosis

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Metabolic Acidosis and Alkalosis

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Mixed Acid-Base Disorders

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Compensatory Mechanisms

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Clinical Assessment of Acid-Base Status

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