Normal pregnancy and antenatal care: amniotic fluid optical density: practice question

A OBG 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.

Try it

OBG · Normal pregnancy and antenatal care: amniotic fluid optical density

Sample item

In a woman with Rh alloimmunisation, the amniotic fluid shows a rise in optical density deviation at 450 nm. Which substance does this reading reflect?

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

The answer and the reasoning

A. Bilirubin from fetal haemolysis

Delta OD 450 measures the absorbance caused by bilirubin in amniotic fluid, which rises with fetal red cell destruction. It was plotted on the Liley chart to grade severity, but middle cerebral artery Doppler has largely replaced this invasive test.

Option by option

  • A. Bilirubin from fetal haemolysis (correct)

    Correct. Bilirubin absorbs light at 450 nm, so the reading follows the degree of haemolysis.

  • B. Biliverdin from fetal liver

    Biliverdin does not accumulate in amniotic fluid in haemolytic disease and does not absorb at 450 nm in this test.

  • C. Urobilinogen from fetal gut

    Urobilinogen is formed in the gut by bacteria and is not the pigment measured here.

  • D. Free haemoglobin from maternal cells

    The reading reflects a breakdown pigment from fetal cells, not free haemoglobin from maternal cells.

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.

More OBG questions

Questions about this OBG question

What is the answer to this OBG question?

The best answer is A, Bilirubin from fetal haemolysis. Delta OD 450 measures the absorbance caused by bilirubin in amniotic fluid, which rises with fetal red cell destruction.

Why is "Biliverdin from fetal liver" not the answer?

Biliverdin does not accumulate in amniotic fluid in haemolytic disease and does not absorb at 450 nm in this test.

Why is "Urobilinogen from fetal gut" not the answer?

Urobilinogen is formed in the gut by bacteria and is not the pigment measured here.

Why is "Free haemoglobin from maternal cells" not the answer?

The reading reflects a breakdown pigment from fetal cells, not free haemoglobin from maternal cells.

What concept does this question test?

It tests Normal pregnancy and antenatal care: amniotic fluid optical density in OBG. It was plotted on the Liley chart to grade severity, but middle cerebral artery Doppler has largely replaced this invasive test.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Normal pregnancy and antenatal care: amniotic fluid optical density?

Learn management algorithms as steps: PPH, eclampsia, ectopic pregnancy. Then use the flashcards and the other OBG 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.