Bacteriology: scrub typhus and Weil-Felix 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.

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Microbiology · Bacteriology: scrub typhus and Weil-Felix test

Sample item

A 38-year-old farmer from a hill district of Himachal Pradesh has fever for 8 days, headache, and a painless black eschar in the axilla with regional lymphadenopathy. Which Weil-Felix antigen is expected to agglutinate with his serum?

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

The answer and the reasoning

C. Proteus OX-K

Scrub typhus is caused by Orientia tsutsugamushi, transmitted by trombiculid mite larvae (chiggers), and produces an eschar. It cross-reacts with Proteus OX-K in the Weil-Felix test, whereas typhus group rickettsiae react with OX-19 and spotted fever group with OX-19 and OX-2. Doxycycline is the drug of choice.

Option by option

  • A. Proteus OX-19

    OX-19 is strongly positive in typhus group infections such as epidemic and murine typhus.

  • B. Proteus OX-2

    OX-2 reacts predominantly in spotted fever group infections.

  • C. Proteus OX-K (correct)

    Correct. OX-K agglutination is the Weil-Felix pattern of scrub typhus.

  • D. Salmonella O antigen 9

    Salmonella O antigens are used in the Widal test for enteric fever, not for rickettsial infection.

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.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Proteus OX-K. Scrub typhus is caused by Orientia tsutsugamushi, transmitted by trombiculid mite larvae (chiggers), and produces an eschar.

Why is "Proteus OX-19" not the answer?

OX-19 is strongly positive in typhus group infections such as epidemic and murine typhus.

Why is "Proteus OX-2" not the answer?

OX-2 reacts predominantly in spotted fever group infections.

Why is "Salmonella O antigen 9" not the answer?

Salmonella O antigens are used in the Widal test for enteric fever, not for rickettsial infection.

What concept does this question test?

It tests Bacteriology: scrub typhus and Weil-Felix test in Microbiology. It cross-reacts with Proteus OX-K in the Weil-Felix test, whereas typhus group rickettsiae react with OX-19 and spotted fever group with OX-19 and OX-2. Doxycycline is the drug of choice.

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 Bacteriology: scrub typhus and Weil-Felix 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.