Calcium and Phosphate Homeostasis — MCQs
In hypoparathyroidism:
The function of which of the following is increased by an elevated parathyroid hormone concentration:
Parathormone has all of the following effects, except -
A patient is on a low calcium diet for 8 weeks. Which of the following increases to maintain serum calcium levels?
Which of the following causes hypocalcemia:
All are examples of negative feedback except
What is the primary effect of GLP-1 on insulin secretion?
The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?
All of the following statements regarding stones in the submandibular gland are true except one:
Which of the following substances is NOT typically found in gallstones?
Calcium and Phosphate Homeostasis Indian Medical PG Practice Questions and MCQs
Question 1: In hypoparathyroidism:
- A. Plasma calcium is high and inorganic phosphorous low
- B. Plasma calcium and inorganic phosphorous are low
- C. Plasma calcium is low and inorganic phosphorous high (Correct Answer)
- D. Plasma calcium and inorganic phosphorous are high
Explanation: ***Plasma calcium is low and inorganic phosphorous high*** - **Hypoparathyroidism** is characterized by insufficient parathyroid hormone (PTH) production, leading to decreased bone resorption and reduced renal reabsorption of calcium [1]. This results in **hypocalcemia** (low plasma calcium) [1]. - PTH also promotes renal excretion of phosphate [2]. With insufficient PTH, renal phosphate excretion is impaired, leading to **hyperphosphatemia** (high inorganic phosphorus) [1]. *Plasma calcium is high and inorganic phosphorous low* - This profile is characteristic of **primary hyperparathyroidism**, where excessive PTH causes increased bone resorption and renal calcium reabsorption (high calcium), and increased renal phosphate excretion (low phosphorus). - It directly contradicts the defining features of hypoparathyroidism [1]. *Plasma calcium and inorganic phosphorous are low* - While plasma calcium is low in hypoparathyroidism, plasma inorganic phosphorus is characteristically high, not low [1]. - A combination of low calcium and low phosphorus can be seen in conditions like **vitamin D deficiency** (osteomalacia), but not directly in pure hypoparathyroidism [1]. *Plasma calcium and inorganic phosphorous are high* - This combination of high calcium and high phosphorus is uncommon and not typically seen in either hypoparathyroidism or hyperparathyroidism. - It could potentially indicate conditions like **milk-alkali syndrome** or **vitamin D intoxication**, but not hypoparathyroidism, which is defined by low calcium [1].
Question 2: The function of which of the following is increased by an elevated parathyroid hormone concentration:
- A. Action of osteoblasts only
- B. Osteoclasts (Correct Answer)
- C. Phosphate reabsorptive pathways in the renal tubules
- D. Hepatic formation of 25-hydroxycholecalciferol
Explanation: ***Osteoclasts*** - **Parathyroid hormone (PTH)** primarily acts to increase serum calcium levels by stimulating **osteoclasts**, leading to bone resorption and release of calcium and phosphate into the bloodstream. - While PTH does not directly act on osteoclasts, it binds to receptors on osteoblasts, which then release factors that activate osteoclasts. *Action of osteoblasts only* - PTH indirectly affects **osteoblasts** by binding to their receptors, but this action primarily leads to **RANKL expression**, which then stimulates osteoclast activity, not a direct increase in osteoblastic bone formation. - Chronic elevation of PTH, as seen in primary hyperparathyroidism, can paradoxically lead to a net loss of bone mass due to increased osteoclastic activity. *Phosphate reabsorptive pathways in the renal tubules* - PTH actually **decreases reabsorption of phosphate** in the renal tubules, leading to phosphaturia. This helps to prevent calcium-phosphate precipitation by lowering serum phosphate levels while raising calcium. - This is a key mechanism by which PTH increases serum calcium—by both mobilizing it from bone and reducing its renal excretion, while simultaneously promoting renal phosphate excretion. *Hepatic formation of 25-hydroxycholecalciferol* - The **liver** is responsible for the hydroxylation of vitamin D3 (cholecalciferol) to **25-hydroxycholecalciferol (calcidiol)**, a process that is not directly regulated by PTH. - PTH primarily stimulates the **kidneys** to convert 25-hydroxycholecalciferol to its active form, **1,25-dihydroxyvitamin D (calcitriol)**, which then enhances intestinal calcium absorption.
Question 3: Parathormone has all of the following effects, except -
- A. Increased calcitriol synthesis
- B. Increased phosphate reabsorption in kidney (Correct Answer)
- C. Increased Ca+2 reabsorption in kidney
- D. Increased bone resorption
Explanation: ***Increased phosphate reabsorption in kidney*** - Parathormone (**PTH**) primarily functions to increase serum **calcium** levels. - One of its key actions is to promote **phosphate excretion** by decreasing phosphate reabsorption in the renal tubules, not increasing it. *Increased calcitriol synthesis* - **PTH** stimulates the renal 1-alpha-hydroxylase enzyme, which is crucial for converting 25-hydroxyvitamin D to its active form, **1,25-dihydroxyvitamin D (calcitriol)**. - This active form of **vitamin D** then enhances intestinal calcium absorption. *Increased Ca+2 reabsorption in kidney* - **PTH** directly acts on the renal tubules, particularly the distal tubule and collecting duct, to increase the **reabsorption of calcium**. - This prevents calcium loss from the body and contributes to raising serum calcium levels. *Increased bone resorption* - **PTH** stimulates osteoclasts, leading to the breakdown of bone and the release of **calcium** and phosphate into the bloodstream. - This process, known as **bone resorption**, is a significant mechanism by which PTH increases serum calcium.
Question 4: A patient is on a low calcium diet for 8 weeks. Which of the following increases to maintain serum calcium levels?
- A. Active 24,25 dihydroxy cholecalciferol
- B. PTH (Correct Answer)
- C. Serum phosphate level
- D. Calcitonin
Explanation: ***PTH*** - **Parathyroid hormone (PTH)** is the primary regulator of calcium homeostasis and the key hormone that **increases in response to hypocalcemia** (low serum calcium). - In a patient on a low calcium diet for 8 weeks, **PTH secretion increases** to maintain normal serum calcium levels. - PTH acts through three main mechanisms: increasing **bone resorption** (releasing calcium from bone), enhancing renal **calcium reabsorption** in the distal tubule, and stimulating the production of **active vitamin D (1,25-dihydroxycholecalciferol)** which increases intestinal calcium absorption. *Active 24,25 dihydroxy cholecalciferol* - **24,25-dihydroxycholecalciferol** is a relatively **inactive metabolite** of vitamin D and represents a pathway of vitamin D catabolism, not activation. - The **active form** of vitamin D that increases calcium absorption is **1,25-dihydroxycholecalciferol (calcitriol)**, whose production is stimulated by PTH. - This metabolite does **not increase** in response to hypocalcemia as a compensatory mechanism. *Serum phosphate level* - A low calcium diet would **not directly lead to an increase in serum phosphate levels**. - In fact, PTH (which increases in response to low calcium) typically causes a **decrease in serum phosphate** by promoting renal phosphate excretion (phosphaturic effect). - High phosphate levels can actually exacerbate hypocalcemia by forming insoluble calcium-phosphate complexes. *Calcitonin* - **Calcitonin** is released from the thyroid parafollicular cells (C cells) in response to **high serum calcium levels** (hypercalcemia). - It acts to **lower** calcium by inhibiting osteoclast activity and reducing renal calcium reabsorption. - In hypocalcemia (low calcium diet), calcitonin secretion would **decrease, not increase**, making this the opposite of what occurs to maintain calcium homeostasis.
Question 5: Which of the following causes hypocalcemia:
- A. 1, 25-dihydroxycholecalciferol
- B. Parathormone
- C. Thyroid hormones
- D. Calcitonin (Correct Answer)
Explanation: ***Calcitonin*** - **Calcitonin** is a hormone secreted by the **parafollicular cells (C cells)** of the thyroid gland. - It **lowers serum calcium levels** by **inhibiting osteoclast activity** (preventing bone resorption) and **increasing renal calcium excretion**. - This is the only hormone among the options that causes hypocalcemia. *1,25-dihydroxycholecalciferol* - This is the **active form of vitamin D** (calcitriol), which **increases serum calcium levels**. - It promotes intestinal calcium absorption, enhances bone resorption, and increases renal calcium reabsorption. - Deficiency of this hormone leads to hypocalcemia, but the hormone itself raises calcium. *Parathormone* - **Parathyroid hormone (PTH)** is the primary regulator that **increases serum calcium levels**. - It stimulates **osteoclast activity** (releasing calcium from bone), increases renal calcium reabsorption, and promotes synthesis of 1,25-dihydroxycholecalciferol. - PTH acts opposite to calcitonin in calcium homeostasis. *Thyroid hormones* - **Thyroxine (T4) and triiodothyronine (T3)** primarily regulate metabolism and have **no direct role in calcium homeostasis**. - While severe thyroid dysfunction can indirectly affect bone turnover, thyroid hormones do not directly cause hypocalcemia.
Question 6: All are examples of negative feedback except
- A. Regulation of blood CO2 level
- B. Regulation of pituitary hormones
- C. Regulation of blood pressure
- D. Coagulation of the blood (Correct Answer)
Explanation: ***Coagulation of the blood*** - **Blood coagulation** is a classic example of **positive feedback**, where the initial clotting process amplifies itself until bleeding stops - Platelets aggregate and release factors that promote further platelet aggregation and activation of the clotting cascade, thereby **accelerating the response** rather than diminishing it - This is the exception among the options, as it represents positive feedback while all others are negative feedback *Regulation of blood CO2 level* - The regulation of **blood CO2 levels** is a vital example of **negative feedback**, where an increase in CO2 stimulates breathing to expel excess CO2 - This mechanism works to return the blood CO2 concentration to its homeostatic set point, thus **counteracting the initial stimulus** - Central and peripheral chemoreceptors detect elevated CO2 and trigger increased ventilation *Regulation of pituitary hormones* - The regulation of **pituitary hormones** involves **negative feedback loops**, where high levels of target gland hormones inhibit the release of stimulating hormones from the pituitary and hypothalamus - For example, high thyroid hormone levels inhibit TSH release from the pituitary and TRH from the hypothalamus - This effectively **reduces the initial stimulus** and maintains hormonal balance *Regulation of blood pressure* - The regulation of **blood pressure** is primarily controlled by **negative feedback mechanisms** involving baroreceptors, which detect changes in pressure - If blood pressure rises, baroreceptors in the carotid sinus and aortic arch signal the medulla to reduce heart rate and dilate blood vessels - This response **lowers the pressure back to the set point**, maintaining cardiovascular homeostasis
Question 7: What is the primary effect of GLP-1 on insulin secretion?
- A. Increased aldosterone secretion by adrenal
- B. Increased PTH secretion
- C. Increased insulin secretion from beta-cells of pancreas (Correct Answer)
- D. Increased testosterone secretion from Leydig cells
Explanation: ***Increased insulin secretion from beta-cells of pancreas*** - **Glucagon-like peptide-1 (GLP-1)** is an **incretin hormone** that stimulates **glucose-dependent insulin secretion** from pancreatic beta-cells. - This effect is crucial for maintaining **glucose homeostasis**, especially after a meal. *Increased aldosterone secretion by adrenal* - **Aldosterone secretion** is primarily regulated by the **renin-angiotensin-aldosterone system (RAAS)** and potassium levels, not directly by GLP-1. - Aldosterone's main function is to regulate **sodium and water balance** and **blood pressure**. *Increased PTH secretion* - **Parathyroid hormone (PTH)** secretion is primarily regulated by **serum calcium levels**. - Its main role is to maintain **calcium homeostasis** by affecting bone, kidney, and intestine. *Increased testosterone secretion from Leydig cells* - **Testosterone secretion** from Leydig cells is primarily regulated by **luteinizing hormone (LH)** from the pituitary gland. - GLP-1 has no direct significant role in **gonadal steroidogenesis**.
Question 8: The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?
- A. NH4OH, pKa = 9.24
- B. Na2HPO4, pKa = 12.32
- C. NaH2PO4, pKa = 7.21 (Correct Answer)
- D. CH3CO2H, pKa = 4.75
Explanation: ***NaH2PO4, pKa = 7.21*** - A buffer's maximum effectiveness is typically within 1 pH unit of its **pKa value**. - With a **pKa of 7.21**, the H2PO4⁻/HPO4²⁻ buffer system (phosphate buffer) is optimally positioned to buffer fluctuations around the physiologic pH of **7.35-7.45**. - This makes the phosphate buffer system highly effective in intracellular and urinary pH regulation. *NH4OH, pKa = 9.24* - This compound is a **weak base** with a pKa of 9.24, meaning it would be effective at a pH much higher than the physiologic range. - Its buffering capacity would be minimal at **pH 7.4**, as the system would be predominantly in one form, reducing its ability to resist pH changes. *Na2HPO4, pKa = 12.32* - This represents the **second dissociation** of phosphoric acid (HPO4²⁻ ⇌ PO4³⁻ + H⁺) with a very high **pKa of 12.32**. - This dissociation occurs at extremely alkaline pH levels, far above the physiological range. - At physiologic pH, this equilibrium would be almost entirely shifted to HPO4²⁻, providing no buffering capacity. *CH3CO2H, pKa = 4.75* - **Acetic acid** has a pKa of 4.75, making it an effective buffer in the acidic range (around pH 3.75-5.75). - It would be almost entirely dissociated at **physiologic pH**, offering very little buffering capacity against pH changes in body fluids.
Question 9: All of the following statements regarding stones in the submandibular gland are true except one:
- A. Majority of submandibular stones are radiolucent (Correct Answer)
- B. Patient presents with acute pain and swelling in the region of the submandibular gland
- C. 80% of stones occur in the submandibular gland
- D. The hypoglossal nerve is at risk during submandibular gland excision
Explanation: ***Majority of submandibular stones are radiolucent*** - This statement is **incorrect** because submandibular stones (sialoliths) are typically composed of **calcium phosphate and calcium carbonate**, making them **radiopaque** on plain radiographs. - Approximately **80-90%** of submandibular stones are radiopaque and visible on X-rays. - Only 10-20% are radiolucent, making this the false statement in this "except" question. *80% of stones occur in the submandibular gland* - This statement is **true**; the submandibular gland is the most common site for salivary stones, accounting for **80-92%** of all sialoliths. - This high prevalence is due to Wharton's duct being longer and more tortuous, gravity-dependent positioning, and the alkaline mucinous nature of submandibular saliva. *Patient presents with acute pain and swelling in the region of the submandibular gland* - This is **true** and represents the classic presentation of sialolithiasis. - Pain and swelling typically **worsen with eating** (when salivary flow increases) and may partially subside between meals. - This is often called "mealtime syndrome" or prandial pain. *The hypoglossal nerve is at risk during submandibular gland excision* - This statement is **true**. During submandibular gland excision, the **hypoglossal nerve (CN XII)** runs close to the superior aspect of the gland and can be injured. - Other nerves at risk include the **marginal mandibular branch of the facial nerve** (most commonly injured, causing lower lip weakness) and the **lingual nerve** (causing tongue numbness). - Hypoglossal nerve injury results in tongue deviation toward the affected side and difficulties with speech and swallowing.
Question 10: Which of the following substances is NOT typically found in gallstones?
- A. Cholesterol
- B. Oxalates (Correct Answer)
- C. Phosphate
- D. Carbonate
Explanation: ***Oxalates*** - Gallstones are primarily composed of **cholesterol**, **bile pigments**, and **calcium salts**, but they do not contain oxalates [1][2][3]. - Oxalates are more commonly associated with **kidney stones**, making this correct. *Carbonate* - Gallstones can contain **calcium carbonate**, particularly in certain types of stones, indicating that this option is incorrect. - These stones are formed in the **gallbladder**, often due to altered bile composition [2]. *Phosphate* - Some gallstones can contain **calcium phosphate**, especially in cases of infection or liver disease, which makes this option unsuitable. - Phosphate can contribute to the formation of **mixed stones** in the gallbladder. *Cholesterol* - In fact, the most common type of gallstone is the **cholesterol stone**, indicating that this option is incorrect [1][3]. - Cholesterol stones form when there is excessive **cholesterol** in the bile, leading to crystallization [3]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, pp. 882-883. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 403-404. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, p. 882.
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