Magnesium Metabolism — MCQs
A woman with eclampsia is started on magnesium sulfate. What is the first sign of magnesium sulfate toxicity?
Which enzyme requires zinc as a cofactor?
Hypomagnesemia due to increased excretion by the kidney is caused by all except:
Hypermagnesemia may be observed in:
Normal anion gap is___ mmol/L?
In Bartter's syndrome there is a defect in
In renal failure, what is the primary cause of metabolic acidosis?
Which of the following statements is not true?
In a child with suspected tetany, the following test is performed. Identify the sign?
Intracellular water constitutes what percentage of total body water?
Magnesium Metabolism Indian Medical PG Practice Questions and MCQs
Question 1: A woman with eclampsia is started on magnesium sulfate. What is the first sign of magnesium sulfate toxicity?
- A. Respiratory depression
- B. Hypotension
- C. Loss of knee jerk (Correct Answer)
- D. Reduced muscle tone
Explanation: ***Loss of knee jerk*** - **Diminished or absent deep tendon reflexes**, particularly the knee jerk, is the **earliest clinical sign** of magnesium sulfate toxicity. - This occurs at serum magnesium levels between **7-10 mEq/L** (8.5-12 mg/dL) due to magnesium's depressant effect on the nervous system and neuromuscular transmission. *Respiratory depression* - **Respiratory depression** is a more severe and later sign of magnesium toxicity, occurring at higher serum levels (typically >12 mEq/L). - It indicates significant central nervous system depression and potential for respiratory arrest, usually after reflexes are already lost. *Hypotension* - While magnesium sulfate can cause **vasodilation** and a subsequent drop in blood pressure, it is generally **not the first sign of toxicity** and often occurs concurrently with other mild to moderate signs. - Hypotension may be part of the therapeutic effect to reduce blood pressure in eclampsia, rather than an initial indicator of toxicity. *Reduced muscle tone* - **Reduced muscle tone** or **flaccidity** is also a consequence of magnesium's neuromuscular blocking effect but typically manifests **after the loss of deep tendon reflexes**. - It signifies more profound neuromuscular impairment, closer to the progression towards respiratory depression.
Question 2: Which enzyme requires zinc as a cofactor?
- A. Lactate dehydrogenase
- B. Carbonic anhydrase (Correct Answer)
- C. Glutathione peroxidase
- D. Hexokinase
Explanation: ***Carbonic anhydrase*** - **Carbonic anhydrase** is a critical enzyme that rapidly interconverts carbon dioxide and water into carbonic acid, which then dissociates into a proton and a bicarbonate ion. - This enzyme contains a **zinc ion** in its active site, which is essential for its catalytic activity, particularly in binding and activating water for the hydration of CO2. *Lactate dehydrogenase* - **Lactate dehydrogenase (LDH)** catalyzes the reversible conversion of pyruvate to lactate, a key step in anaerobic glycolysis. - LDH primarily relies on **NAD+ or NADH** as cofactors and does not require zinc. *Glutathione peroxidase* - **Glutathione peroxidase (GPx)** is an antioxidant enzyme that catalyzes the reduction of hydrogen peroxide and organic hydroperoxides to water using glutathione. - Most mammalian glutathione peroxidases are **selenium-dependent enzymes**, incorporating selenocysteine at their active site, rather than zinc. *Hexokinase* - **Hexokinase** is an enzyme that phosphorylates hexoses, most notably glucose, to glucose-6-phosphate, the first step in glycolysis. - This enzyme requires **magnesium (Mg2+)** as a cofactor for its activity, as it forms a complex with ATP, facilitating the transfer of the phosphate group.
Question 3: Hypomagnesemia due to increased excretion by the kidney is caused by all except:
- A. Aminoglycoside
- B. Furosemide
- C. Cisplatin
- D. Digitalis (Correct Answer)
Explanation: ***Digitalis*** - **Digitalis** (digoxin) is a cardiac glycoside that inhibits the **Na+/K+-ATPase pump**; it is not directly associated with renal magnesium wasting. Instead, its toxicity can be exacerbated by hypomagnesemia, but it does not cause it. - While it affects electrolyte balance intracellularly, it does not primarily lead to increased **urinary excretion of magnesium**. *Aminoglycoside* - **Aminoglycosides** like gentamicin can cause **renal tubular damage**, particularly in the distal tubules. - This damage impairs the kidney's ability to reabsorb magnesium, leading to increased **urinary magnesium excretion** and hypomagnesemia. *Furosemide* - **Furosemide** is a **loop diuretic** that inhibits the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle. - This inhibition reduces the **transepithelial potential difference**, which in turn reduces the passive reabsorption of magnesium and calcium, leading to increased urinary excretion. *Cisplatin* - **Cisplatin** is a chemotherapeutic agent known to cause **renal tubular toxicity**, particularly affecting the distal convoluted tubule. - This toxicity often results in impaired magnesium reabsorption and consequently increased **urinary magnesium loss**.
Question 4: Hypermagnesemia may be observed in:
- A. Hypothyroidism
- B. Primary hypoparathyroidism
- C. Acute Kidney Injury (Correct Answer)
- D. Adrenal insufficiency
Explanation: ***Acute Kidney Injury*** - When the kidneys are unable to adequately excrete excess magnesium, it accumulates in the body, leading to **hypermagnesemia**. - This is a common cause of hypermagnesemia, especially in patients who are also receiving **magnesium-containing medications** (e.g., antacids, laxatives). *Hypothyroidism* - Hypothyroidism is typically associated with **hypo**magnesemia due to altered renal handling and increased urinary excretion. - It is also commonly linked with **hypo**calcemia. *Primary hypoparathyroidism* - Primary hypoparathyroidism is characterized by **decreased parathyroid hormone (PTH)**, leading to **hypocalcemia** [1] and often **hyperphosphatemia**. - Magnesium levels are typically normal or slightly reduced, as PTH plays a role in magnesium reabsorption in the tubule [2]. *Adrenal insufficiency* - Adrenal insufficiency (Addison's disease) is characterized by a deficiency in mineralocorticoids, leading to **hyponatremia** and **hyperkalemia**. - Magnesium levels are usually normal or can be slightly elevated due to hemoconcentration, but it is not a direct cause of significant hypermagnesemia.
Question 5: Normal anion gap is___ mmol/L?
- A. 8-16 (Correct Answer)
- B. 30-34
- C. 20-24
- D. 0-4
Explanation: ***8-16*** - The normal range for the **anion gap** is generally considered to be 8-16 mmol/L, reflecting the unmeasured anions in the plasma. - This range can vary slightly between laboratories, but **8-16 mmol/L** is the most commonly accepted range in clinical practice. *30-34* - This range is significantly **higher than normal** and would indicate a **high anion gap metabolic acidosis**, rather than a normal anion gap. - A high anion gap suggests an accumulation of **unmeasured acids** in the body, such as in lactic acidosis or ketoacidosis. *20-24* - This value is also **elevated** compared to the normal range, suggesting a high anion gap. - An anion gap in this range would prompt investigation into causes of **metabolic acidosis** with an increased anion gap. *0-4* - This range is significantly **lower than normal** and could indicate a **low or negative anion gap**, which is a rare finding. - A low anion gap is often associated with conditions like **hypoalbuminemia**, multiple myeloma (due to paraproteins), or severe hypernatremia.
Question 6: In Bartter's syndrome there is a defect in
- A. Descending limb of LOH
- B. Thick ascending limb of LOH (Correct Answer)
- C. DCT
- D. PCT
Explanation: ***Thick ascending limb of LOH*** - **Bartter's syndrome** is characterized by a genetic defect affecting the **Na-K-2Cl cotransporter (NKCC2)** located in the thick ascending limb of the loop of Henle. - This defect impairs the reabsorption of sodium, potassium, and chloride ions, leading to significant **electrolyte imbalances** such as hypokalemia, metabolic alkalosis, and hyperreninemia. *Descending limb of LOH* - The descending limb is primarily permeable to **water** due to aquaporin channels, and impermeable to solutes. - Defects in this segment are not typically associated with the electrolyte derangements seen in Bartter's syndrome. *DCT* - The **distal convoluted tubule (DCT)** is where fine-tuning of sodium and calcium reabsorption occurs, primarily through the Na-Cl cotransporter (NCC) and active calcium transport. - Defects in the DCT are characteristic of **Gitelman's syndrome**, which has similar but generally milder symptoms compared to Bartter's syndrome. *PCT* - The **proximal convoluted tubule (PCT)** is responsible for the bulk reabsorption of filtered substances, including glucose, amino acids, bicarbonate, and about 65-70% of filtered sodium. - While defects here can lead to various syndromes (e.g., Fanconi syndrome), they do not directly cause the specific electrolyte abnormalities seen in Bartter's syndrome.
Question 7: In renal failure, what is the primary cause of metabolic acidosis?
- A. Use of diuretics
- B. Loss of HCO3-
- C. Increased H+ production
- D. Decreased excretion of acids (Correct Answer)
Explanation: ***Decreased excretion of acids*** - In **renal failure**, the kidneys lose their ability to effectively excrete metabolic acid byproducts, leading to their accumulation in the body. - This accumulation of acids, such as **sulfates**, **phosphates**, and **urea**, consumes bicarbonate buffers, resulting in metabolic acidosis. *Increased H+ production* - While overproduction of **H+ ions** can cause acidosis, like in **ketoacidosis** or **lactic acidosis**, it's not the primary underlying mechanism in most cases of renal failure. - The problem in renal failure is primarily one of **impaired elimination**, not excessive generation, of acids. *Loss of HCO3-* - Loss of **bicarbonate (HCO3-)** can occur in conditions like severe diarrhea or renal tubular acidosis, but it's not the primary cause of metabolic acidosis in general renal failure. - In renal failure, decreased **ammoniagenesis** and impaired reabsorption of bicarbonate can contribute, but the main driver is reduced acid excretion. *Use of diuretics* - The use of **diuretics** (especially loop or thiazide diuretics) typically causes **metabolic alkalosis** due to increased potassium and hydrogen ion excretion, rather than acidosis. - Some diuretics, like **carbonic anhydrase inhibitors**, can cause a mild metabolic acidosis, but this is less common and not the primary cause of renal failure-associated acidosis.
Question 8: Which of the following statements is not true?
- A. Mg2+ deficiency impairs PTH secretion and can lead to hypocalcemia
- B. Parathyroid hormone-related protein is responsible for causing hypercalcemia in cancer patients
- C. Ca2+ influences PTH secretion by acting on a calcium sensor G-protein coupled receptor located in the parathyroid gland
- D. The unionized fraction of calcium in the plasma is an important determinant of PTH secretion (Correct Answer)
Explanation: ***The unionized fraction of calcium in the plasma is an important determinant of PTH secretion*** - The **ionized (free)** fraction of calcium, not the unionized fraction, is the physiologically active form that is critical for regulating **PTH secretion** and other cellular processes. - The parathyroid glands respond to the level of ionized calcium in the extracellular fluid to maintain **calcium homeostasis**. - This is the **FALSE statement** because unionized calcium is not the active determinant. *Mg2+ deficiency impairs PTH secretion and can lead to hypocalcemia* - **Hypomagnesemia** impairs PTH secretion and also causes target tissue resistance to PTH. - This leads to **hypocalcemia** that is difficult to correct until magnesium levels are restored. - Magnesium is a necessary cofactor for normal parathyroid function. *Parathyroid hormone-related protein is responsible for causing hypercalcemia in cancer patients* - **Parathyroid hormone-related protein (PTHrP)** is indeed a common cause of **humoral hypercalcemia of malignancy (HHM)**, mimicking the actions of PTH and leading to high calcium levels in cancer patients. - Many tumors, particularly squamous cell carcinomas, produce PTHrP, which binds to **PTH receptors** in bone and kidneys, resulting in increased bone resorption and renal calcium reabsorption. *Ca2+ influences PTH secretion by acting on a calcium sensor G-protein coupled receptor located in the parathyroid gland* - **Calcium (Ca2+)** directly regulates PTH secretion via the **calcium-sensing receptor (CaSR)**, which is a G-protein coupled receptor located on the chief cells of the parathyroid glands. - When **extracellular ionized calcium levels** are high, CaSR is activated, leading to inhibition of PTH secretion; conversely, low calcium levels reduce CaSR activation, stimulating PTH release.
Question 9: In a child with suspected tetany, the following test is performed. Identify the sign?
- A. Chvostek sign
- B. Allen sign
- C. Trousseau sign (Correct Answer)
- D. Turner sign
Explanation: ***Trousseau sign*** - The image depicts a blood pressure cuff inflated on the arm, leading to **carpopedal spasm** in the hand, which is characteristic of the **Trousseau sign**. - This sign is indicative of **latent tetany** and is often seen in conditions causing **hypocalcemia**. *Chvostek sign* - The Chvostek sign involves a **facial muscle twitch** elicited by tapping the facial nerve anterior to the ear. - This sign is also associated with hypocalcemia but differs clinically from the presentation in the image. *Allen sign* - The Allen test (not "sign") is performed to assess the **patency of the ulnar and radial arteries** before arterial puncture or cannulation. - It involves digitally compressing both arteries and observing the return of color to the hand after releasing one artery, which is unrelated to the image. *Turner sign* - The Turner sign refers to **flank ecchymosis** (bruising) and is a physical finding associated with **hemorrhagic pancreatitis**. - This sign indicates retroperitoneal bleeding, which is not represented by the image or related to tetany.
Question 10: Intracellular water constitutes what percentage of total body water?
- A. 25%
- B. 80%
- C. 40%
- D. 60% (Correct Answer)
Explanation: ***60%*** - **Intracellular fluid (ICF)** makes up approximately **two-thirds (67%)** of the total body water. - Among the given options, **60% is the closest approximation** to the actual value. - ICF refers to the fluid contained within cells, crucial for mediating cellular reactions and maintaining cell volume. - ICF comprises about **40% of total body weight** (67% of 60% TBW). *40%* - This represents the approximate percentage of **total body weight** that is intracellular water, not the percentage of total body water. - As a proportion of total body water, ICF is much higher (approximately 67%). *25%* - This value is significantly lower than the actual proportion of intracellular water. - No major fluid compartment accounts for 25% of total body water. *80%* - This percentage is much higher than the actual proportion of intracellular water. - An 80% proportion would be physiologically inconsistent with normal fluid distribution between ICF and ECF compartments.
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