Calcium and phosphate handling US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Calcium and phosphate handling. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Calcium and phosphate handling US Medical PG Question 1: A 65-year-old female with chronic renal failure presents with recent onset of bone pain. Serum analysis reveals decreased levels of calcium and elevated levels of parathyroid hormone. One of the mechanisms driving the elevated PTH is most similar to that seen in:
- A. Decreased functioning of the calcium-sensing receptor (CASR)
- B. Sarcoidosis
- C. Parathyroid adenoma
- D. End stage liver failure
- E. Insufficient Ca intake (Correct Answer)
Calcium and phosphate handling Explanation: ***Insufficient Ca intake***
- **Chronic renal failure** impairs the kidney's ability to activate **vitamin D**, leading to **decreased calcium absorption** from the gut.
- This resulting **hypocalcemia** stimulates the parathyroid glands to increase **PTH secretion** to normalize calcium, a response similar to that seen with insufficient dietary calcium intake.
*Decreased functioning of the calcium-sensing receptor (CASR)*
- A dysfunctional CASR would lead to **hypercalcemia** and inappropriately normal or elevated PTH, as the parathyroid gland would not recognize normal or high calcium levels.
- In chronic renal failure, the primary issue is **hypocalcemia** leading to secondary hyperparathyroidism, not a problem with calcium sensing itself.
*Sarcoidosis*
- Sarcoidosis can cause **hypercalcemia** due to extrarenal production of **1,25-dihydroxyvitamin D** (calcitriol) by activated macrophages.
- This condition would typically lead to **suppressed PTH levels**, which is the opposite of the elevated PTH seen in the patient's case.
*Parathyroid adenoma*
- A parathyroid adenoma causes **primary hyperparathyroidism**, characterized by **hypercalcemia** and inappropriately high PTH levels.
- The patient in the vignette presents with **hypocalcemia**, making primary hyperparathyroidism less likely as the direct cause of the elevated PTH.
*End stage liver failure*
- Liver failure can impair **vitamin D hydroxylation** in the liver, leading to **decreased 25-hydroxyvitamin D** levels.
- While this can contribute to hypocalcemia and elevated PTH (secondary hyperparathyroidism), it is distinct from the primary mechanism in renal failure involving the kidney's crucial role in **1-alpha hydroxylation** of vitamin D.
Calcium and phosphate handling US Medical PG Question 2: Which region of the nephron reabsorbs the highest percentage of filtered bicarbonate?
- A. Collecting duct
- B. Thick ascending limb
- C. Distal tubule
- D. Proximal tubule (Correct Answer)
Calcium and phosphate handling Explanation: ***Proximal tubule***
- The **proximal convoluted tubule (PCT)** reabsorbs approximately 80-90% of the **filtered bicarbonate** through a process involving **carbonic anhydrase** and the **Na+/H+ exchanger**.
- This vital function ensures that the majority of bicarbonate, a key buffer, is returned to the blood to maintain **acid-base balance**.
*Collecting duct*
- While the collecting duct does have the ability to reabsorb and secrete bicarbonate, its contribution is minor compared to the PCT, primarily for fine-tuning acid-base balance.
- Cells in the collecting duct, particularly **Type A intercalated cells**, are important for secreting acid (H+) in acidosis and therefore reabsorbing bicarbonate, but not the bulk of it.
*Thick ascending limb*
- The primary role of the **thick ascending limb** is the reabsorption of **sodium**, **potassium**, and **chloride** to create a concentrated interstitium, not significant bicarbonate reabsorption.
- It is largely impermeable to water and is relatively impermeable to bicarbonate.
*Distal tubule*
- The **distal convoluted tubule (DCT)** reabsorbs a small percentage of filtered bicarbonate, but its main role is regulated reabsorption of **sodium** and **calcium**, and secretion of **potassium** and **hydrogen ions**.
- Its contribution to bicarbonate reabsorption is much less significant than that of the proximal tubule.
Calcium and phosphate handling US Medical PG Question 3: A 55-year-old man with long-standing diabetes presents with a fragility fracture. He has chronic renal failure secondary to his diabetes. His serum parathyroid hormone concentration is elevated. You measure his serum concentration of 25(OH)-vitamin D and find it to be normal, but his concentration of 1,25(OH)-vitamin D is decreased. Which of the following represents a correct pairing of his clinical condition and serum calcium level?
- A. Secondary hyperparathyroidism with elevated serum calcium
- B. Primary hyperparathyroidism with low serum calcium
- C. Tertiary hyperparathyroidism with low serum calcium
- D. Secondary hyperparathyroidism with low serum calcium (Correct Answer)
- E. Primary hyperparathyroidism with elevated serum calcium
Calcium and phosphate handling Explanation: ***Secondary hyperparathyroidism with low serum calcium***
- **Chronic renal failure** leads to reduced 1-alpha hydroxylation in the kidneys, decreasing the conversion of 25(OH)-vitamin D to its active form, **1,25(OH)-vitamin D**.
- This **decrease in active vitamin D** impairs intestinal calcium absorption and causes resistance to PTH in bone, resulting in **hypocalcemia**, which then stimulates the parathyroid glands to produce more PTH, leading to **secondary hyperparathyroidism**.
*Secondary hyperparathyroidism with elevated serum calcium*
- In secondary hyperparathyroidism due to chronic renal failure, the aim is to correct **hypocalcemia**, so a persistently elevated serum calcium would suggest a different or progressing pathology, such as tertiary hyperparathyroidism.
- While PTH is elevated, it is primarily trying to normalize calcium, which is typically low or low-normal, not elevated, in uncomplicated secondary hyperparathyroidism.
*Primary hyperparathyroidism with low serum calcium*
- **Primary hyperparathyroidism** is characterized by excessive PTH production from a parathyroid adenoma or hyperplasia and typically results in **hypercalcemia**, not low serum calcium.
- The elevated PTH in primary hyperparathyroidism is autonomous, not provoked by hypocalcemia, and would also lead to elevated 1,25(OH)-vitamin D levels.
*Tertiary hyperparathyroidism with low serum calcium*
- **Tertiary hyperparathyroidism** occurs when prolonged secondary hyperparathyroidism leads to autonomous PTH secretion, often resulting in **hypercalcemia**, not hypocalcemia, despite the underlying renal failure.
- This option incorrectly pairs tertiary hyperparathyroidism with low serum calcium; the key feature of tertiary hyperparathyroidism is the emancipation of PTH secretion from calcium regulation.
*Primary hyperparathyroidism with elevated serum calcium*
- While **primary hyperparathyroidism** typically presents with elevated serum calcium, this patient has **chronic renal failure** and a decreased 1,25(OH)-vitamin D, which points to a renal etiology for the calcium imbalance.
- In primary hyperparathyroidism, 1,25(OH)-vitamin D levels would generally be normal or elevated due to PTH stimulation of renal 1-alpha hydroxylase, which contradicts the patient's decreased levels.
Calcium and phosphate handling US Medical PG Question 4: Which factor most strongly influences protein filtration at the glomerulus?
- A. Electrical charge
- B. Molecular size (Correct Answer)
- C. Shape
- D. Temperature
Calcium and phosphate handling Explanation: ***Molecular size***
- The glomerular filtration barrier, particularly the **slit diaphragms** between podocytes, acts as a size-selective filter, restricting the passage of larger molecules.
- Proteins like **albumin** (molecular radius ~36 Å, molecular weight ~69 kDa) are significantly large, making them difficult to pass through the filtration barrier.
- Size selectivity is the **primary and most important** factor in protein filtration.
*Electrical charge*
- The glomerular basement membrane contains **negatively charged proteoglycans** (heparan sulfate), which repel negatively charged proteins like albumin, contributing to their retention.
- While important, the role of electrical charge is **secondary** to molecular size in preventing the bulk passage of most proteins.
*Shape*
- While abnormal protein shapes (e.g., **amyloid fibrils**) can impact filtration in specific disease states, the typical physiological filtration of most proteins is primarily governed by size and charge.
- The inherent shape of normal globular proteins plays a less direct role compared to their overall size.
*Temperature*
- **Physiological temperature** is relatively constant in the body and does not directly influence the molecular interactions and physical properties of the glomerular filtration barrier in a way that significantly alters protein filtration.
- Temperature changes would lead to denaturation or aggregation, which are not the primary determinants of normal protein filtration.
Calcium and phosphate handling US Medical PG Question 5: A 76-year-old woman presents to the office with a generalized weakness for the past month. She has a past medical history significant for uncontrolled hypertension and type 2 diabetes mellitus. Her temperature is 37.0°C (98.6°F), blood pressure is 135/82 mm Hg, pulse is 90/min, respiratory rate is 17/min, and oxygen saturation is 99% on room air. Physical exam shows no remarkable findings. Her last recorded glomerular filtration rate was 30 mL/min. A radiograph of the patient’s hand is given. Which of the following lab findings is most likely to be found in this patient?
- A. Increased PTH, decreased calcium, increased phosphate (Correct Answer)
- B. Increased PTH, decreased calcium, decreased phosphate
- C. Normal PTH, increased calcium, normal phosphate
- D. Increased PTH, increased calcium, decreased phosphate
- E. Increased PTH, increased calcium, increased phosphate
Calcium and phosphate handling Explanation: ***Increased PTH, decreased calcium, increased phosphate***
- The patient's **glomerular filtration rate (GFR) of 30 mL/min** indicates **Stage 4 chronic kidney disease (CKD)**. In CKD, the kidneys are unable to adequately excrete phosphate, leading to **hyperphosphatemia**.
- Hyperphosphatemia leads to the formation of calcium-phosphate complexes, causing a decrease in free calcium (hypocalcemia) by precipitating out. Additionally, damaged kidneys cannot convert vitamin D into its active form, which further reduces calcium absorption from the gut and bone, contributing to hypocalcemia. This persistent hypocalcemia stimulates the parathyroid glands to produce more **parathyroid hormone (PTH)**, resulting in **secondary hyperparathyroidism**.
*Increased PTH, decreased calcium, decreased phosphate*
- This pattern of laboratory findings is characteristic of **primary hyperparathyroidism**, where an adenoma or hyperplasia of the parathyroid glands leads to excessive PTH production.
- In primary hyperparathyroidism, elevated PTH causes increased bone resorption and renal phosphate excretion, typically resulting in **hypercalcemia** and **hypophosphatemia**.
*Normal PTH, increased calcium, normal phosphate*
- This combination is not typically seen in CKD-related mineral and bone disorder.
- While hypercalcemia can occur in some conditions, normal PTH with increased calcium often points toward conditions like **paraneoplastic syndromes** or **granulomatous diseases** producing PTH-related protein, or **vitamin D intoxication**.
*Increased PTH, increased calcium, decreased phosphate*
- This specific combination (high PTH, high calcium, low phosphate) is the classic hallmark of **primary hyperparathyroidism**.
- In contrast, secondary hyperparathyroidism due to CKD typically presents with **hypocalcemia** and **hyperphosphatemia** due to impaired renal function.
*Increased PTH, increased calcium, increased phosphate*
- This pattern is highly unusual and not characteristic of any common parathyroid or kidney disorder.
- While both PTH and phosphate can be increased in certain advanced stages of CKD, calcium levels are typically low or normal, not elevated in secondary hyperparathyroidism.
Calcium and phosphate handling US Medical PG Question 6: An investigator is studying the effects of hyperphosphatemia on calcium homeostasis. A high-dose phosphate infusion is administered intravenously to a healthy subject over the course of 3 hours. Which of the following sets of changes is most likely to occur in response to the infusion?
$$$ Serum parathyroid hormone %%% Serum total calcium %%% Serum calcitriol %%% Urine phosphate $$$
- A. ↑ ↓ ↓ ↑
- B. ↓ ↑ ↑ ↓
- C. ↑ ↑ ↑ ↑
- D. ↓ ↓ ↓ ↓
- E. ↑ ↑ ↑ ↓
- F. ↑ ↓ ↑ ↑ (Correct Answer)
Calcium and phosphate handling Explanation: ***↑ ↓ ↑ ↑***
- A high-dose phosphate infusion causes **hyperphosphatemia**, which leads to binding of ionized calcium and results in **hypocalcemia** (decreased serum total calcium).
- The hypocalcemia stimulates the parathyroid glands to secrete **increased PTH**.
- Elevated PTH stimulates 1α-hydroxylase in the kidneys, leading to **increased calcitriol** (active vitamin D) production to enhance intestinal calcium absorption and renal calcium reabsorption.
- Both the high filtered load of phosphate and **PTH's phosphaturic effect** lead to **increased urinary phosphate excretion** as the kidneys attempt to restore phosphate balance.
*↑ ↓ ↓ ↑*
- This option correctly predicts increased PTH and increased urinary phosphate, but incorrectly suggests **decreased calcitriol**.
- PTH stimulation would increase 1α-hydroxylase activity, leading to **increased calcitriol production**, not decreased.
*↓ ↑ ↑ ↓*
- This option incorrectly predicts **decreased PTH** following hyperphosphatemia.
- Hyperphosphatemia causes hypocalcemia, which **stimulates PTH release**, not suppresses it.
- An increase in serum total calcium is also incorrect, as phosphate binds calcium acutely.
*↑ ↑ ↑ ↑*
- While this option correctly predicts increased PTH and calcitriol, it incorrectly suggests **increased serum total calcium**.
- Acute hyperphosphatemia causes calcium-phosphate binding, leading to **decreased ionized and total calcium**, which is the trigger for PTH release.
- However, urinary phosphate would correctly increase in this scenario.
*↓ ↓ ↓ ↓*
- This option is completely incorrect as it suggests all parameters decrease.
- Hyperphosphatemia triggers compensatory mechanisms including **increased PTH and calcitriol**, not decreases.
- Urinary phosphate must **increase** to excrete the excess phosphate load, not decrease.
*↑ ↑ ↑ ↓*
- This option incorrectly combines increased serum total calcium with **decreased urinary phosphate**.
- In hyperphosphatemia, urinary phosphate excretion **must increase** due to both the filtered load and PTH's phosphaturic effect.
- Additionally, acute phosphate binding would **decrease** serum calcium initially, though compensatory mechanisms attempt to restore it.
Calcium and phosphate handling US Medical PG Question 7: A 55-year-old woman presents to the emergency room with severe abdominal pain for the past 24 hours. She has also noticed blood in her urine. She does not have any significant past medical history. Family history is significant for her mother having cholecystitis status post cholecystectomy at age 45. Her vital signs include: temperature 36.8°C (98.2°F), pulse 103/min, respiratory rate 15/min, blood pressure 105/85 mm Hg. Physical examination is significant for a woman continuously moving on the exam table in an attempt to get comfortable. Laboratory findings are significant for the following:
Serum electrolytes
Na 138 mEq/L N: 135–145 mEq/L
K 4.0 mEq/L N: 3.5–5.0 mEq/L
Cl 102 mEq/L N: 98–108 mEq/L
CO2 27 mEq/L N: 22–32 mEq/L
Ca 9.2 mEq/dL N: 8.4–10.2 mEq/dL
PO4 3.5 mg/dL N: 3.0–4.5 mg/dL
A 24-hour urine collection is performed and reveals a urinary calcium of 345 mg/day (ref: < 300 mg/day in men; < 250 mg/day in women). A non-contrast CT of the abdomen is performed and is shown in the exhibit. The patient’s symptoms pass within the next 12 hours with hydration and acetaminophen for pain management. She is prescribed a medication to prevent subsequent episodes. At which of the following parts of the nephron does this medication most likely work?
- A. Descending limb of the loop of Henle
- B. Distal convoluted tubule (Correct Answer)
- C. Proximal tubule
- D. Collecting ducts
- E. Thick ascending limb of the loop of Henle
Calcium and phosphate handling Explanation: **Distal convoluted tubule**
- The patient's symptoms (severe abdominal pain, hematuria, flank movement, and high urinary calcium) and CT findings (likely urinary calculi) point to **nephrolithiasis** (kidney stones).
- The medication prescribed to prevent subsequent episodes would most likely be a **thiazide diuretic**, which acts on the **distal convoluted tubule** to increase calcium reabsorption and decrease urinary calcium excretion.
*Descending limb of the loop of Henle*
- This segment is primarily responsible for **water reabsorption** and is permeable to water but impermeable to solutes, playing no significant role in the reabsorption of calcium that would be targeted by stone prevention medication.
- No common diuretic for nephrolithiasis prevention directly targets calcium handling in this part of the nephron.
*Proximal tubule*
- While a significant amount of calcium is reabsorbed here, medications specifically used to prevent kidney stones (like thiazides) do not primarily act on the **proximal tubule** for their calcium-sparing effects.
- The primary function of the proximal tubule is **bulk reabsorption** of filtered solutes and water.
*Collecting ducts*
- The collecting ducts are primarily involved in **fine-tuning water reabsorption** under the influence of ADH and also regulate potassium and acid-base balance.
- While some calcium reabsorption can occur here, it's not the main site of action for **thiazide diuretics** used in kidney stone prevention.
*Thick ascending limb of the loop of Henle*
- This segment is responsible for reabsorbing significant amounts of sodium, potassium, and chloride, and is the site of action for **loop diuretics**.
- **Loop diuretics** actually *increase* urinary calcium excretion and are thus contraindicated in calcium kidney stone prevention.
Calcium and phosphate handling US Medical PG Question 8: In a healthy patient with no renal abnormalities, several mechanisms are responsible for moving various filtered substances into and out of the tubules. Para-aminohippurate (PAH) is frequently used to estimate renal blood flow when maintained at low plasma concentrations. The following table illustrates the effect of changing plasma PAH concentrations on PAH excretion:
Plasma PAH concentration (mg/dL) | Urinary PAH concentration (mg/dL)
0 | 0
10 | 60
20 | 120
30 | 150
40 | 180
Which of the following mechanisms best explains the decreased rate of increase in PAH excretion observed when plasma PAH concentration exceeds 20 mg/dL?
- A. Decreased glomerular filtration of PAH
- B. Increased rate of PAH reabsorption
- C. Increased flow rate of tubular contents
- D. Saturation of PAH transport carriers (Correct Answer)
- E. Increased diffusion rate of PAH
Calcium and phosphate handling Explanation: ***Saturation of PAH transport carriers***
- PAH is primarily cleared by **tubular secretion** via organic anion transporters (OATs) in the proximal tubule, which have a **finite transport maximum (Tm)**.
- When plasma PAH concentration exceeds the capacity of these carriers (as seen above 20 mg/dL), the transporters become saturated, leading to a **decreased incremental excretion** despite rising plasma levels.
*Decreased glomerular filtration of PAH*
- **Glomerular filtration rate (GFR)** for PAH is proportional to its plasma concentration and is typically constant in a healthy kidney, so it would not decrease with increasing plasma PAH.
- A decrease in GFR would lead to a *reduced* overall excretion, but not specifically explain the *decreased rate of increase* at higher plasma concentrations.
*Increased rate of PAH reabsorption*
- PAH is **minimally reabsorbed** in the renal tubules; its primary mechanism of removal from the blood is active secretion.
- An increase in reabsorption would lead to *less* PAH in the urine, but there's no physiological basis for increased reabsorption as plasma concentration rises.
*Increased flow rate of tubular contents*
- While an increased flow rate can sometimes affect solute reabsorption or secretion, it would generally lead to a more, not less, efficient clearance of secreted substances.
- This mechanism does not explain the **saturation kinetics** observed with PAH at higher plasma concentrations.
*Increased diffusion rate of PAH*
- PAH is a charged organic anion, and its movement across tubular membranes is primarily mediated by **active transport** rather than simple diffusion.
- Even if diffusion played a minor role, an increased diffusion rate would generally lead to *more* excretion, not the observed plateau in the rate of increase.
Calcium and phosphate handling US Medical PG Question 9: An investigator is studying membranous transport proteins in striated muscle fibers of an experimental animal. An electrode is inserted into the gluteus maximus muscle and a low voltage current is applied. In response to this, calcium is released from the sarcoplasmic reticulum of the muscle fibers and binds to troponin C, which results in a conformational change of tropomyosin and unblocking of the myosin-binding site. The membranous transport mechanism underlying the release of calcium into the cytosol most resembles which of the following processes?
- A. Opening of acetylcholine receptors at neuromuscular junction
- B. Reabsorption of glucose by renal tubular cells
- C. Secretion of doxorubicin from dysplastic colonic cells
- D. Uptake of fructose by small intestinal enterocytes (Correct Answer)
- E. Removal of calcium from cardiac myocytes
Calcium and phosphate handling Explanation: **Uptake of fructose by small intestinal enterocytes**
- The release of calcium from the sarcoplasmic reticulum into the cytosol in muscle contraction is primarily mediated by **ryanodine receptors**, which are a type of **facilitated diffusion channel**.
- **Fructose uptake** in the small intestine occurs via **GLUT5 transporters**, which also utilize **facilitated diffusion**, moving fructose down its concentration gradient without direct energy expenditure.
*Opening of acetylcholine receptors at neuromuscular junction*
- The opening of **acetylcholine receptors** is a form of **ligand-gated ion channel** activity, specific to the binding of acetylcholine.
- While it involves channel opening, it's initiated by a chemical signal, whereas sarcoplasmic reticulum calcium release is often voltage-gated or mechanically coupled to voltage sensors.
*Reabsorption of glucose by renal tubular cells*
- Glucose reabsorption in renal tubules primarily involves **secondary active transport** via **SGLT transporters**, which co-transport glucose with sodium.
- This process requires energy indirectly, unlike the facilitated diffusion of calcium from the sarcoplasmic reticulum.
*Secretion of doxorubicin from dysplastic colonic cells*
- The secretion of doxorubicin, an anticancer drug, from cells often involves **ATP-binding cassette (ABC) transporters** (e.g., MDR1), which utilize **primary active transport** to pump substances against their concentration gradient using ATP.
- This is an energy-dependent process, distinct from facilitated diffusion.
*Removal of calcium from cardiac myocytes*
- The removal of calcium from cardiac myocytes occurs primarily via the **SERCA pump** (an **ATP-dependent active transporter**) back into the sarcoplasmic reticulum and the **Na+/Ca2+ exchanger** (a **secondary active transporter**) out of the cell.
- Both mechanisms require energy, either directly or indirectly, to move calcium against its electrochemical gradient.
Calcium and phosphate handling US Medical PG Question 10: A 52-year-old man comes to the physician because of a 4-month history of fatigue, weakness, constipation, decreased appetite, and intermittent flank pain. He takes ibuprofen for knee and shoulder pain. Physical examination shows mild tenderness bilaterally in the costovertebral areas. His serum calcium concentration is 11.2 mg/dL, phosphorus concentration is 2.5 mg/dL, and N-terminal parathyroid hormone concentration is 830 pg/mL. Which of the following steps in vitamin D metabolism is most likely increased in this patient?
- A. Ergocalciferol → 25-hydroxyergocalciferol
- B. 7-dehydrocholesterol → cholecalciferol
- C. 25-hydroxycholecalciferol → 1,25-dihydroxycholecalciferol (Correct Answer)
- D. 25-hydroxycholecalciferol → 24,25-dihydroxycholecalciferol
- E. Cholecalciferol → 25-hydroxycholecalciferol
Calcium and phosphate handling Explanation: ***25-hydroxycholecalciferol → 1,25-dihydroxycholecalciferol***
- This patient presents with **hypercalcemia** (11.2 mg/dL), **hypophosphatemia** (2.5 mg/dL), and a markedly **elevated N-terminal parathyroid hormone (PTH)** concentration (830 pg/mL), which are classic findings for **primary hyperparathyroidism**.
- In primary hyperparathyroidism, elevated PTH directly stimulates the **renal 1-alpha-hydroxylase enzyme**, increasing the conversion of **25-hydroxycholecalciferol** (calcidiol) to its active form, **1,25-dihydroxycholecalciferol** (calcitriol), which raises calcium levels.
*Ergocalciferol → 25-hydroxyergocalciferol*
- This step involves the **hepatic 25-hydroxylase enzyme** converting dietary vitamin D2 (ergocalciferol) to its storage form, which is not primarily regulated by PTH in the context of hyperparathyroidism.
- While essential for vitamin D activation, this conversion rate is usually adequate and not the primary increased step responsible for the hypercalcemic state in this patient's presentation.
*7-dehydrocholesterol → cholecalciferol*
- This process is the **cutaneous synthesis of vitamin D3 (cholecalciferol)**, which is dependent on UV light exposure and is not directly regulated by PTH.
- This initial step of vitamin D synthesis occurs in the skin and is upstream of the metabolic pathway influenced by PTH.
*25-hydroxycholecalciferol → 24,25-dihydroxycholecalciferol*
- This conversion produces an **inactive form of vitamin D** and is catalyzed by the **24-hydroxylase enzyme**.
- This enzyme activity is typically **suppressed by high PTH** and **increased by high levels of 1,25-dihydroxycholecalciferol**, serving to degrade excess active vitamin D; therefore, this step would likely be decreased, not increased, in primary hyperparathyroidism.
*Cholecalciferol → 25-hydroxycholecalciferol*
- This is the **hepatic 25-hydroxylation** of vitamin D3, producing 25-hydroxycholecalciferol (calcidiol), the major circulating form of vitamin D.
- While critical for producing the substrate for further activation, this step is not the *most likely increased* step in response to high PTH in primary hyperparathyroidism.
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