Acid-Base Balance Practice Questions

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

Ammonia. ### 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.

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

Respiratory alkalosis. ### 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 ( 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.

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

Respiratory acidosis. **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.

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

Hyperchloremic acidosis. **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.

Q: What is the primary determinant of the anion gap?

Proteins. **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).

Q: Why is tetany seen with hyperventilation?

Respiratory alkalosis. ### 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.

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

Histamine. ### 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).

Question 1

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

Accepted Answer

Ureterosigmoidostomy

Answer

Thiazide diuretic therapy

Answer

Prolonged vomiting

Answer

Cushing's disease

Question 2

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

Accepted Answer

Ammonia

Answer

Bicarbonate

Answer

Phosphate

Answer

Protein

Question 3

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

Accepted Answer

Respiratory alkalosis

Answer

Respiratory acidosis

Answer

Metabolic alkalosis

Answer

Metabolic acidosis

Question 4

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

Accepted Answer

Respiratory acidosis

Answer

Metabolic acidosis

Answer

Respiratory alkalosis

Answer

Excessive renal bicarbonate loss

Question 5

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

Accepted Answer

Hyperchloremic acidosis

Answer

Lactic acidosis

Answer

Ketoacidosis

Answer

Methanol poisoning

Question 6

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

Accepted Answer

Secretion of Ammonia

Answer

Secretion of bicarbonate

Answer

Secretion of HCl

Answer

Absorption of Ammonia

Question 7

What is the primary determinant of the anion gap?

Accepted Answer

Proteins

Answer

Sulphates

Answer

Phosphates

Answer

Nitrates

Question 8

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

Accepted Answer

Decreased pH with decreased HCO3-

Answer

Increased pH with increased HCO3-

Answer

Increased pH with decreased HCO3-

Answer

Decreased pH with increased HCO3-

Question 9

Why is tetany seen with hyperventilation?

Accepted Answer

Respiratory alkalosis

Answer

Metabolic alkalosis

Answer

Metabolic acidosis

Answer

Respiratory acidosis

Question 10

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

Accepted Answer

Histamine

Answer

Acetylcholine

Answer

Noradrenaline

Answer

Dopamine

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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