Electrolytes: SIADH: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Electrolytes: SIADH

Sample item

A 62-year-old smoker with small cell lung carcinoma is euvolaemic with serum sodium 120 mmol/L and serum osmolality 255 mOsm/kg. Which urine findings confirm SIADH?

AUrine osmolality below serum osmolality with high urine volumeBUrine sodium below 10 mmol/L with clinical dehydrationCUrine osmolality above 100 mOsm/kg with urine sodium above 30 to 40 mmol/LDUrine osmolality below 100 mOsm/kg with low urine sodium

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Urine osmolality above 100 mOsm/kg with urine sodium above 30 to 40 mmol/L

In SIADH, antidiuretic hormone is inappropriately high despite hypo-osmolality, so urine is not maximally dilute (above 100 mOsm/kg) and sodium excretion continues (usually above 30 to 40 mmol/L) in a euvolaemic patient. Primary polydipsia gives maximally dilute urine, and hypovolaemia from vomiting gives low urine sodium.

Option by option

A. Urine osmolality below serum osmolality with high urine volume
Dilute high-volume urine would suggest water excess or diabetes insipidus.
B. Urine sodium below 10 mmol/L with clinical dehydration
Low urine sodium with dehydration indicates extrarenal volume loss.
C. Urine osmolality above 100 mOsm/kg with urine sodium above 30 to 40 mmol/L (correct)
Correct. Inappropriately concentrated urine with ongoing sodium loss defines SIADH.
D. Urine osmolality below 100 mOsm/kg with low urine sodium
Maximally dilute urine suggests primary polydipsia, not SIADH.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Urine osmolality above 100 mOsm/kg with urine sodium above 30 to 40 mmol/L. In SIADH, antidiuretic hormone is inappropriately high despite hypo-osmolality, so urine is not maximally dilute (above 100 mOsm/kg) and sodium excretion continues (usually above 30 to 40 mmol/L) in a euvolaemic patient.

Why is "Urine osmolality below serum osmolality with high urine volume" not the answer?

Dilute high-volume urine would suggest water excess or diabetes insipidus.

Why is "Urine sodium below 10 mmol/L with clinical dehydration" not the answer?

Low urine sodium with dehydration indicates extrarenal volume loss.

Why is "Urine osmolality below 100 mOsm/kg with low urine sodium" not the answer?

Maximally dilute urine suggests primary polydipsia, not SIADH.

What concept does this question test?

It tests Electrolytes: SIADH in Biochemistry. Primary polydipsia gives maximally dilute urine, and hypovolaemia from vomiting gives low urine sodium.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Electrolytes: SIADH?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.