Laboratory diagnosis: Limulus amebocyte lysate test: practice question

A Microbiology 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

Microbiology · Laboratory diagnosis: Limulus amebocyte lysate test

Sample item

Several patients develop rigors and fever during an infusion from one batch of intravenous fluid. Cultures of the sterile fluid are negative. Which in vitro test detects the likely cause?

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

The answer and the reasoning

C. Limulus amebocyte lysate assay

Pyrogenic reactions can be caused by endotoxin (lipopolysaccharide) from Gram-negative bacteria that persists after the bacteria are killed or filtered. The Limulus amebocyte lysate (LAL) test uses horseshoe crab lysate that gels in the presence of endotoxin. Culture detects only viable organisms.

Option by option

  • A. Gram stain of the fluid

    A Gram stain misses small amounts of endotoxin and cannot show it.

  • B. Culture on thioglycollate broth

    Thioglycollate broth grows viable organisms and misses residual endotoxin.

  • C. Limulus amebocyte lysate assay (correct)

    Correct. LAL gelation detects bacterial endotoxin even when no live bacteria remain.

  • D. Latex agglutination for Staphylococcus aureus

    Latex agglutination identifies staphylococcal antigens, not endotoxin.

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 Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Limulus amebocyte lysate assay. Pyrogenic reactions can be caused by endotoxin (lipopolysaccharide) from Gram-negative bacteria that persists after the bacteria are killed or filtered.

Why is "Gram stain of the fluid" not the answer?

A Gram stain misses small amounts of endotoxin and cannot show it.

Why is "Culture on thioglycollate broth" not the answer?

Thioglycollate broth grows viable organisms and misses residual endotoxin.

Why is "Latex agglutination for Staphylococcus aureus" not the answer?

Latex agglutination identifies staphylococcal antigens, not endotoxin.

What concept does this question test?

It tests Laboratory diagnosis: Limulus amebocyte lysate test in Microbiology. The Limulus amebocyte lysate (LAL) test uses horseshoe crab lysate that gels in the presence of endotoxin. Culture detects only viable organisms.

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 Laboratory diagnosis: Limulus amebocyte lysate test?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology 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.