Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance Indian Medical PG Practice Questions and MCQs

Question 51: Which of the following is a cause of hyperkalemia?

A. Exercise (Correct Answer)
B. Alkalosis
C. Insulin injection
D. Decreased serum osmolarity

Explanation: **Explanation:** **Correct Answer: A. Exercise** Hyperkalemia occurs during exercise due to the repeated depolarization of skeletal muscle cells. During an action potential, potassium ($K^+$) exits the cell. In vigorous exercise, the rate of $K^+$ efflux exceeds the capacity of the $Na^+/K^+$-ATPase pump to re-uptake it, leading to a transient increase in extracellular potassium. Additionally, exercise-induced minor cell trauma and local acidosis can further shift $K^+$ out of the cells. **Analysis of Incorrect Options:** * **B. Alkalosis:** In alkalotic states, hydrogen ions ($H^+$) move out of cells to compensate for the high pH. To maintain electroneutrality, $K^+$ moves **into** the cells, resulting in **hypokalemia**. * **C. Insulin injection:** Insulin stimulates the $Na^+/K^+$-ATPase pump in skeletal muscle and liver cells, driving $K^+$ **into** the intracellular compartment. This is why insulin (with glucose) is a standard treatment for acute hyperkalemia. * **D. Decreased serum osmolarity:** Hypertonicity (increased osmolarity) causes water to leave cells via osmosis. This "solvent drag" and the resulting increase in intracellular $K^+$ concentration (which favors diffusion out of the cell) cause hyperkalemia. Conversely, **decreased** osmolarity tends to keep $K^+$ inside the cell. **High-Yield Clinical Pearls for NEET-PG:** * **"ASID" (Acidosis, Sugar/Insulin deficiency, Isotonicity/Hyperosmolarity, Digoxin/Drugs):** Common causes of $K^+$ shifting out of cells. * **Beta-blockers** can cause hyperkalemia, while **Beta-agonists** (like Salbutamol) cause hypokalemia by stimulating the $Na^+/K^+$ pump. * **Pseudohyperkalemia:** Always consider hemolysis during blood collection or extreme leukocytosis/thrombocytosis as a cause of falsely elevated $K^+$ levels.

Question 52: What is the ideal biologic solvent?

A. Water (Correct Answer)
B. Coconut oil
C. Ethyl alcohol
D. Liquefied nitrogen

Explanation: **Explanation:** **Water (Option A)** is the ideal biologic solvent because of its unique chemical structure and physical properties. It is a **polar molecule** with a high **dielectric constant**, which allows it to dissolve a wide variety of charged (ionic) and polar (hydrophilic) substances. This property is essential for biochemical reactions, as it facilitates the transport of nutrients, gases, and metabolic waste products across cell membranes and through the bloodstream. Furthermore, water’s high specific heat capacity helps in maintaining homeostatic body temperature. **Why other options are incorrect:** * **Coconut oil (Option B):** This is a non-polar lipid. While it can dissolve fat-soluble vitamins, it cannot dissolve the electrolytes and proteins necessary for cellular metabolism. * **Ethyl alcohol (Option C):** Although it has polar characteristics, it acts as a protein denaturant and disrupts lipid bilayers, making it toxic to cells at high concentrations. * **Liquefied nitrogen (Option D):** This exists only at extremely low temperatures (-196°C). At physiological temperatures, it is a gas and cannot serve as a liquid medium for biochemical reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Total Body Water (TBW):** Approximately 60% of body weight in adult males (50% in females). * **Amphipathic Molecules:** These contain both hydrophobic and hydrophilic regions (e.g., phospholipids). In water, they spontaneously form **micelles** or **bilayers**, which is the structural basis of cell membranes. * **Nucleophilic Attack:** Water is not just a passive solvent; it is a reactant in **hydrolysis** reactions, which are fundamental to the digestion of macronutrients and the breakdown of ATP for energy.

Question 53: Anion gap is increased in all the following conditions except?

A. Renal tubular acidosis (Correct Answer)
B. Diabetic ketoacidosis
C. Acute tubular necrosis
D. Ethylene glycol poisoning

Explanation: The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. It represents unmeasured anions in the plasma. Metabolic acidosis is broadly classified into **High Anion Gap Metabolic Acidosis (HAGMA)** and **Normal Anion Gap Metabolic Acidosis (NAGMA)**. ### Why Renal Tubular Acidosis (RTA) is the Correct Answer: RTA is a classic cause of **NAGMA (Hyperchloremic metabolic acidosis)**. In RTA, there is either a failure to excrete $H^+$ or a failure to reabsorb $HCO_3^-$. To maintain electroneutrality as bicarbonate is lost, the kidneys retain Chloride ($Cl^-$). Since the increase in chloride offsets the decrease in bicarbonate, the calculated Anion Gap remains within the normal range (8–12 mEq/L). ### Why the Other Options are Incorrect: * **Diabetic Ketoacidosis (DKA):** Accumulation of unmeasured ketoacids (acetoacetate and beta-hydroxybutyrate) increases the anion gap. * **Acute Tubular Necrosis (ATN):** In advanced renal failure, the kidneys fail to excrete fixed acids like phosphates and sulfates, leading to HAGMA. * **Ethylene Glycol Poisoning:** Metabolism of ethylene glycol produces glycolic and oxalic acids, which are unmeasured anions that significantly elevate the anion gap. ### High-Yield Clinical Pearls for NEET-PG: * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA (Normal Gap):** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diuretics (Acetazolamide), **RTA**, Pancreatic fistula). * **Gold Standard:** If a question mentions "Hyperchloremic acidosis," always look for RTA or Diarrhea as the answer.

Question 54: Hamann's solution contains all of the following, EXCEPT:

A. Na+
B. Ca++
C. Mg++ (Correct Answer)
D. Lactate

Explanation: **Explanation:** Hamann’s solution is a specialized physiological salt solution used primarily in medical research and clinical biochemistry to study metabolic processes and maintain tissue viability. It is categorized as a balanced electrolyte solution, similar in principle to Ringer’s Lactate, but with a specific composition. **Why Mg++ is the correct answer:** Hamann’s solution specifically lacks **Magnesium (Mg++)**. Its primary components are designed to mimic the extracellular fluid's osmotic and ionic balance without the inclusion of divalent magnesium ions. In contrast, it relies on Sodium, Potassium, Calcium, and Chloride to maintain membrane potential and cellular integrity. **Analysis of Incorrect Options:** * **Na+ (Sodium):** This is the primary cation in Hamann’s solution, essential for maintaining osmolality and the electrochemical gradient. * **Ca++ (Calcium):** Calcium is a critical component of the solution, necessary for maintaining cell-to-cell adhesion and signaling during physiological experiments. * **Lactate:** Like Ringer’s Lactate, Hamann’s solution utilizes lactate as a buffering agent. Lactate is metabolized into bicarbonate in the body, helping to maintain a stable pH and prevent acidosis. **Clinical Pearls for NEET-PG:** * **Composition Check:** Hamann’s solution contains NaCl, KCl, $CaCl_2$, and Sodium Lactate. * **Comparison:** Unlike **Krebs-Henseleit buffer** (which contains MgSO₄) or **Tyrode’s solution** (which contains $MgCl_2$), Hamann’s is notable for the absence of Magnesium. * **High-Yield Fact:** In acid-base biochemistry, remember that **Lactate** is a "potential bicarbonate." It is often preferred over bicarbonate in IV fluids because it is more stable during storage. * **Mnemonics:** Associate "Hamann’s" with "Minimalist"—it lacks the "M" (Magnesium).

Question 55: What is the normal serum calcium level?

A. 4-6 mg/dL
B. 9-11 mg/dL (Correct Answer)
C. 19-21 mg/dL
D. 20-30 mg/dL

Explanation: **Explanation:** The normal range for total serum calcium in a healthy adult is **9 to 11 mg/dL** (or 2.2 to 2.6 mmol/L). Calcium is a vital divalent cation essential for bone mineralization, blood coagulation, nerve impulse transmission, and muscle contraction. In the blood, calcium exists in three forms: 1. **Ionized (Free) Calcium (~50%):** The physiologically active form. 2. **Protein-bound (~40%):** Mostly bound to albumin. 3. **Complexed (~10%):** Bound to anions like citrate or phosphate. **Analysis of Options:** * **Option A (4-6 mg/dL):** This range is significantly low. While it approximates the normal range for *ionized* calcium (4.5–5.5 mg/dL), it is life-threateningly low for *total* serum calcium (hypocalcemia). * **Options C & D (19-30 mg/dL):** These values represent extreme, lethal hypercalcemia. Levels above 14 mg/dL are considered a "hypercalcemic crisis," often associated with malignancy or severe hyperparathyroidism. **High-Yield Clinical Pearls for NEET-PG:** * **Albumin Correction:** Since 40% of calcium is bound to albumin, the "corrected calcium" must be calculated if albumin is low. * *Formula:* Corrected Ca = Measured Ca + [0.8 × (4.0 - Serum Albumin)]. * **Hormonal Regulation:** Serum calcium is strictly regulated by **Parathyroid Hormone (PTH)** (increases Ca²⁺), **Vitamin D** (increases Ca²⁺), and **Calcitonin** (decreases Ca²⁺). * **Acid-Base Link:** Alkalosis increases calcium binding to albumin, decreasing ionized calcium and leading to tetany, even if total calcium is normal.

Question 56: Anion gap is increased in all the following conditions except?

A. Salicylate toxicity
B. Renal failure
C. Lactic acidosis
D. Diarrhoea (Correct Answer)

Explanation: **Explanation:** The **Anion Gap (AG)** is calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. It represents unmeasured anions in the plasma (like albumin, phosphates, and organic acids). Metabolic acidosis is classified into two types based on this gap: **High Anion Gap Metabolic Acidosis (HAGMA)** and **Normal Anion Gap Metabolic Acidosis (NAGMA).** **Why Diarrhoea is the correct answer:** Diarrhoea is a classic cause of **NAGMA** (Hyperchloremic metabolic acidosis). In diarrhoea, there is a direct loss of bicarbonate ($HCO_3^-$) from the lower GI tract. To maintain electroneutrality, the kidneys retain Chloride ($Cl^-$). Since the decrease in bicarbonate is offset by an equal increase in chloride, the calculated Anion Gap remains within the normal range (8–12 mEq/L). **Analysis of Incorrect Options (Causes of HAGMA):** * **Salicylate toxicity:** Aspirin overdose leads to the accumulation of exogenous organic acids (salicylates) and interferes with mitochondrial function, increasing the anion gap. * **Renal failure:** In advanced renal failure (Uremia), the kidneys fail to excrete fixed acids like phosphates, sulfates, and urates. These "unmeasured anions" increase the anion gap. * **Lactic acidosis:** Occurs due to tissue hypoxia or sepsis. The accumulation of lactate (an unmeasured anion) replaces bicarbonate, leading to HAGMA. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA:** **USED CARP** (Ureterosigmoidostomy, Small bowel fistula, Extra chloride, Diarrhoea, Carbonic anhydrase inhibitors, Renal tubular acidosis, Pancreatic fistula). * **Key Distinction:** If the question mentions **Renal Tubular Acidosis (RTA)** or **Diarrhoea**, always think **Normal Anion Gap.**

Question 57: What is the most readily detected clinical sign of hypermagnesemia?

A. Drowsiness
B. Hypotension
C. Cerebellar ataxia
D. Disappearance of deep tendon reflexes (Correct Answer)

Explanation: **Explanation:** Hypermagnesemia is a rare but potentially life-threatening electrolyte imbalance, most commonly seen in patients with renal failure or those receiving intravenous magnesium therapy (e.g., for eclampsia). Magnesium acts as a natural calcium channel blocker and inhibits the release of acetylcholine at the neuromuscular junction. **Why the correct answer is right:** The **disappearance of deep tendon reflexes (DTRs)** is the earliest and most reliable clinical sign of hypermagnesemia, typically occurring at serum levels of **4–6 mEq/L** (normal: 1.5–2.5 mEq/L). Because magnesium interferes with neuromuscular transmission, the loss of the patellar reflex serves as a critical "warning sign" before more severe respiratory or cardiac complications occur. **Analysis of incorrect options:** * **A. Drowsiness:** While CNS depression and lethargy occur as levels rise (usually >6 mEq/L), they are non-specific and appear after the loss of reflexes. * **B. Hypotension:** Magnesium causes peripheral vasodilation, leading to hypotension. However, this is often inconsistent and less easily "detected" as a definitive early sign compared to the physical exam finding of areflexia. * **C. Cerebellar ataxia:** This is not a classic feature of hypermagnesemia; neuromuscular blockade presents as weakness or paralysis rather than incoordination. **High-Yield NEET-PG Pearls:** * **Sequence of Toxicity:** Loss of DTRs (4-6 mEq/L) → Respiratory depression (8-10 mEq/L) → Cardiac arrest in diastole (>15 mEq/L). * **ECG Changes:** Similar to hyperkalemia (prolonged PR interval, widened QRS, and peaked T-waves). * **Antidote:** **10% Calcium gluconate** (IV) is the immediate treatment to antagonize the membrane effects of magnesium. * **Monitoring:** In patients on $MgSO_4$ (e.g., PIH/Eclampsia), clinicians must monitor **urine output, respiratory rate, and patellar reflexes.**

Question 58: A 2-year-old child is being evaluated for persistent metabolic acidosis. Blood tests show Na+ 140 mEq/L, K+ 3 mEq/L, Ca2+ 8 mg/L, Mg+2 mEq/L, phosphate 3 mEq/L, pH 7.22, bicarbonate 16 mEq/L, and chloride 112 mEq/L. What is the plasma anion gap?

A. 9
B. 15 (Correct Answer)
C. 22
D. 25

Explanation: ### Explanation **1. Understanding the Correct Answer (Option B: 15)** The **Plasma Anion Gap (AG)** is a calculated value used to differentiate causes of metabolic acidosis. It represents the "unmeasured anions" in the plasma (such as albumin, phosphate, and organic acids). The standard formula for calculating the Anion Gap is: **AG = [Na⁺] – ([Cl⁻] + [HCO₃⁻])** Plugging in the values from the question: * Na⁺ = 140 mEq/L * Cl⁻ = 112 mEq/L * HCO₃⁻ = 16 mEq/L * **AG = 140 – (112 + 16) = 140 – 128 = 12 mEq/L** *Note on the Answer Key:* While the calculated value is **12**, in clinical practice and NEET-PG questions, the normal range is typically **8–12 mEq/L**. However, some textbooks include Potassium in the formula: **(Na⁺ + K⁺) – (Cl⁻ + HCO₃⁻)**. Using this formula: **(140 + 3) – (112 + 16) = 143 – 128 = 15.** Given the options provided, **15** is the intended correct answer based on the inclusion of Potassium. **2. Why Other Options are Incorrect** * **Option A (9):** This would represent a low-normal anion gap, often seen in hypoalbuminemia, which is not supported by the clinical picture of acidosis. * **Options C (22) and D (25):** These represent a **High Anion Gap Metabolic Acidosis (HAGMA)**, seen in conditions like DKA, Lactic Acidosis, or Salicylate poisoning. The calculation here does not reach these values. **3. Clinical Pearls for NEET-PG** * **Normal Anion Gap Metabolic Acidosis (NAGMA):** Also called hyperchloremic acidosis. Common causes include Diarrhea and Renal Tubular Acidosis (RTA). * **High Anion Gap Metabolic Acidosis (HAGMA):** Remember the mnemonic **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Albumin Correction:** For every 1 g/dL decrease in serum albumin below 4 g/dL, the "normal" AG decreases by approximately 2.5 mEq/L. Always check albumin levels in a patient with a low AG.

Question 59: What is a true function of restriction endonucleases?

A. They cut both strands of double-stranded DNA.
B. They produce sticky ends.
C. They produce blunt ends.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** Restriction endonucleases (REs), often referred to as "molecular scissors," are essential enzymes in recombinant DNA technology. They function by recognizing specific palindromic sequences in double-stranded DNA (dsDNA) and cleaving the phosphodiester backbone. 1. **Why Option D is Correct:** * **Cutting both strands (Option A):** By definition, endonucleases cleave internal phosphodiester bonds on **both strands** of the DNA molecule. * **Sticky ends (Option B):** Many REs (e.g., *EcoRI*) make staggered cuts, leaving short, single-stranded overhangs known as "sticky" or cohesive ends. These are highly useful in cloning as they easily re-anneal with complementary sequences. * **Blunt ends (Option C):** Some REs (e.g., *SmaI*) cut straight across the DNA at the same position on both strands, resulting in "blunt" ends. While harder to ligate, they are versatile because any blunt end can be joined to another. Since REs can perform all these functions depending on the specific enzyme used, "All of the above" is the correct choice. 2. **Analysis of Options:** Options A, B, and C are all characteristic features of different classes of restriction enzymes. Selecting only one would be incomplete, as the question asks for a "true function," and all three descriptions are fundamentally accurate. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** REs are naturally found in bacteria, where they serve as a defense mechanism against viral (bacteriophage) DNA. * **Nomenclature:** The first letter is the Genus, the next two are the species, and the Roman numeral indicates the order of discovery (e.g., *EcoRI* from *E. coli*). * **Type II REs:** These are the most commonly used in labs because they cut specifically within or at a fixed distance from their recognition site and do not require ATP. * **Application:** Used in RFLP (Restriction Fragment Length Polymorphism) for DNA fingerprinting and prenatal diagnosis of genetic disorders like Sickle Cell Anemia.

Question 60: What is the osmolarity of standard Oral Rehydration Solution (ORS)?

A. 245 mOsm/L
B. 311 mOsm/L (Correct Answer)
C. 330 mOsm/L
D. 210 mOsm/L

Explanation: The correct answer is **311 mOsm/L**, which refers to the **Standard (Old) WHO-ORS** formulation. ### 1. Understanding the Correct Answer The Standard WHO-ORS was designed to provide optimal glucose-coupled sodium transport to treat dehydration. Its total osmolarity of **311 mOsm/L** is derived from the following composition: * **Sodium Chloride:** 3.5 g/L * **Potassium Chloride:** 1.5 g/L * **Trisodium Citrate:** 2.9 g/L * **Glucose (Anhydrous):** 20 g/L This formulation is slightly hyperosmolar compared to plasma (~285–295 mOsm/L). ### 2. Analysis of Incorrect Options * **Option A (245 mOsm/L):** This is the osmolarity of the **Reduced Osmolarity ORS** (New WHO-ORS), currently recommended for treating diarrhea in children. It reduces stool output and the need for IV fluids. * **Option C (330 mOsm/L):** This was the osmolarity of some older, pre-WHO formulations which were found to be too hypertonic, potentially worsening osmotic diarrhea. * **Option D (210 mOsm/L):** This value is too low and does not correspond to any standard WHO-recommended ORS formulation. ### 3. High-Yield Clinical Pearls for NEET-PG * **Reduced Osmolarity ORS (245 mOsm/L):** This is the current "Gold Standard." Its composition is: Na+ (75), Cl- (65), Glucose (75), K+ (20), and Citrate (10) mmol/L. * **Glucose-Sodium Ratio:** In both formulations, the ratio is approximately **1:1**, which is essential for the SGLT-1 transporter in the small intestine. * **Citrate vs. Bicarbonate:** Trisodium citrate is preferred over bicarbonate because it increases the shelf life of the ORS packet and helps correct metabolic acidosis. * **Re-Su-Mal:** A special ORS for Severely Malnourished children has a lower sodium (45 mmol/L) and higher potassium (40 mmol/L) content.

Practice by Chapter

Acid-Base Chemistry and Buffers

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pH Regulation in Body Fluids

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Respiratory Regulation of Acid-Base Balance

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Renal Regulation of Acid-Base Balance

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Respiratory and Metabolic Acidosis

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Respiratory and Metabolic Alkalosis

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Mixed Acid-Base Disorders

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Interpretation of Arterial Blood Gases

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Electrolyte Homeostasis

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Sodium and Water Balance

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Potassium Balance

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Calcium and Phosphate Metabolism

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