Question:
Blood examination of a patient revealed low serum calcium, elevated phosphorus and elevated PTH. Which of the following investigations is least contributory to the diagnosis?
Blood examination of a patient revealed low serum calcium, elevated phosphorus and elevated PTH. Which of the following investigations is least contributory to the diagnosis?
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Which listed test has nothing to do with the calcium-phosphate-PTH axis?
Updated On: Jun 24, 2026
- Urine myoglobin
- cAMP response to PTH
- Vitamin D levels
- Serum creatinine levels
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The Correct Option is A
Solution and Explanation
Step 1: Interpret the biochemistry.
Low serum calcium with HIGH phosphate and HIGH PTH points to PTH resistance, i.e. pseudohypoparathyroidism. The kidney and bone fail to respond to PTH despite raised hormone levels.
Step 2: Evaluate which tests confirm or refine this diagnosis.
cAMP response to PTH (the Ellsworth-Howard test) is the classic confirmatory test, since the renal tubular cAMP and phosphaturic response is blunted in pseudohypoparathyroidism. Vitamin D levels are helpful because vitamin D deficiency also raises PTH but typically gives LOW (not high) phosphate, so it helps differentiate. Serum creatinine helps exclude chronic kidney disease, another cause of low Ca with high phosphate and high PTH (secondary hyperparathyroidism).
Step 3: Identify the least contributory test.
Urine myoglobin reflects skeletal muscle breakdown (rhabdomyolysis) and has no role in evaluating calcium-phosphate-PTH disorders. It contributes nothing to this diagnosis.
Step 4: Recall-key correction.
The printed answer key marks Vitamin D levels, but vitamin D testing IS contributory because it distinguishes vitamin D deficiency from PTH resistance. The genuinely non-contributory test is urine myoglobin, so the medically correct answer is option A.
Conclusion: Urine myoglobin is the least contributory investigation.
Low serum calcium with HIGH phosphate and HIGH PTH points to PTH resistance, i.e. pseudohypoparathyroidism. The kidney and bone fail to respond to PTH despite raised hormone levels.
Step 2: Evaluate which tests confirm or refine this diagnosis.
cAMP response to PTH (the Ellsworth-Howard test) is the classic confirmatory test, since the renal tubular cAMP and phosphaturic response is blunted in pseudohypoparathyroidism. Vitamin D levels are helpful because vitamin D deficiency also raises PTH but typically gives LOW (not high) phosphate, so it helps differentiate. Serum creatinine helps exclude chronic kidney disease, another cause of low Ca with high phosphate and high PTH (secondary hyperparathyroidism).
Step 3: Identify the least contributory test.
Urine myoglobin reflects skeletal muscle breakdown (rhabdomyolysis) and has no role in evaluating calcium-phosphate-PTH disorders. It contributes nothing to this diagnosis.
Step 4: Recall-key correction.
The printed answer key marks Vitamin D levels, but vitamin D testing IS contributory because it distinguishes vitamin D deficiency from PTH resistance. The genuinely non-contributory test is urine myoglobin, so the medically correct answer is option A.
Conclusion: Urine myoglobin is the least contributory investigation.
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