CSR Bone and Mineral Metabolism in CKD (CKD-MBD) 1 — Questions and Answers
Question 1: What is the primary mechanism by which CKD leads to secondary hyperparathyroidism?
- Increased intestinal calcium absorption stimulates PTH release
- Phosphate retention reduces ionized calcium and directly stimulates PTH secretion (Correct answer)
- Decreased sodium reabsorption triggers compensatory PTH release
- Elevated calcitriol levels suppress calcium-sensing receptor activity
Correct answer: Phosphate retention reduces ionized calcium and directly stimulates PTH secretion
In CKD, reduced phosphate excretion causes hyperphosphatemia, which lowers ionized calcium and suppresses calcitriol production, both of which stimulate the parathyroid glands to secrete more PTH.
Question 2: Which form of vitamin D is synthesized by the kidney and considered the biologically active form used in CKD management?
- Ergocalciferol (vitamin D2)
- Cholecalciferol (vitamin D3)
- Calcidiol (25-hydroxyvitamin D)
- Calcitriol (1,25-dihydroxyvitamin D) (Correct answer)
Correct answer: Calcitriol (1,25-dihydroxyvitamin D)
Calcitriol (1,25-dihydroxyvitamin D) is produced by 1-alpha hydroxylase in the kidney and is the most potent active form; its synthesis is impaired in CKD, directly contributing to CKD-MBD.
Question 3: FGF-23 (fibroblast growth factor 23) rises early in CKD progression. What are its two primary effects on mineral metabolism?
- Increases intestinal phosphate absorption and stimulates PTH secretion
- Suppresses renal phosphate reabsorption and stimulates calcitriol synthesis
- Increases urinary phosphate excretion and suppresses renal calcitriol production (Correct answer)
- Stimulates osteoblast activity and reduces serum calcium
Correct answer: Increases urinary phosphate excretion and suppresses renal calcitriol production
FGF-23 promotes phosphaturia by downregulating NaPi-2a/2c co-transporters and suppresses 1-alpha hydroxylase, reducing calcitriol synthesis, which collectively worsen secondary hyperparathyroidism.
Question 4: According to KDIGO 2017 guidelines, what is the recommended target for intact PTH (iPTH) in dialysis patients (CKD G5D)?
- Normal range: 10–65 pg/mL
- 1–2 times the upper limit of normal
- 2–9 times the upper limit of normal (Correct answer)
- Greater than 10 times the upper limit of normal
Correct answer: 2–9 times the upper limit of normal
KDIGO 2017 recommends maintaining iPTH between 2 and 9 times the upper limit of normal in dialysis patients, balancing the risks of adynamic bone disease (over-suppression) against osteitis fibrosa (under-treatment).
Question 5: Which type of renal osteodystrophy is characterized by low bone turnover, reduced osteoblast and osteoclast activity, and is associated with over-suppression of PTH or aluminum toxicity?
- Osteitis fibrosa cystica
- Mixed uremic osteodystrophy
- Adynamic bone disease (Correct answer)
- Osteomalacia
Correct answer: Adynamic bone disease
Adynamic bone disease features minimal bone remodeling due to very low PTH levels or prior aluminum accumulation, increasing fracture risk and the risk of hypercalcemia from calcium loading.
Question 6: The phosphorus-to-protein ratio (mg phosphorus per gram of protein) is an important counseling tool. Which protein source generally has the LOWEST phosphorus-to-protein ratio?
- Processed cheese
- Dark cola beverages
- Egg whites (Correct answer)
- Whole milk
Correct answer: Egg whites
Egg whites provide high-quality protein with very low phosphorus content (approximately 4 mg phosphorus per gram of protein), making them ideal for CKD patients requiring protein while limiting phosphate.
Question 7: In CKD patients, phosphorus from plant-based foods (e.g., legumes, grains) is generally absorbed at what rate compared to phosphorus from animal-based foods?
- The same rate — all dietary phosphorus is equally absorbed
- Higher rate — plant enzymes release extra phosphorus
- Lower rate — plant phosphorus is bound to phytate and requires phytase for release (Correct answer)
- Not clinically relevant — only phosphate binder timing matters
Correct answer: Lower rate — plant phosphorus is bound to phytate and requires phytase for release
Phosphorus in plant foods is predominantly stored as phytate, which humans cannot efficiently digest due to limited phytase activity, resulting in only 20–40% bioavailability compared to 40–60% from animal sources.
CSR Bone and Mineral Metabolism in CKD (CKD-MBD) 1
What is the primary mechanism by which CKD leads to secondary hyperparathyroidism?