Bacteriology: Salmonella Typhi: 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: Salmonella Typhi

Sample item

A 24-year-old man has had fever for 12 days and took oral amoxicillin and ofloxacin for the last 5 days from a local practitioner without relief. Enteric fever is suspected, but a blood culture drawn today is sterile. Which investigation is most likely to confirm the diagnosis?

AA single Widal test at this visitBUrine cultureCRepeat stool cultureDBone marrow culture

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

The answer and the reasoning

D. Bone marrow culture

In enteric fever the bacilli are in the reticuloendothelial system, so bone marrow culture remains positive in most patients even after several days of antibiotics and late in the illness, when blood culture sensitivity has fallen. Blood culture is the best test in the first week, while stool and urine cultures become positive only in the second and third weeks and are less sensitive. A single Widal titre in an endemic area has poor specificity.

Option by option

A. A single Widal test at this visit
A single titre cannot be interpreted in an endemic area because of background antibodies; a rising titre on paired sera is needed, and this is a serological test, not a confirmation of the organism.
B. Urine culture
Urine culture is positive only in a minority of patients in the later weeks and is much less sensitive than marrow culture.
C. Repeat stool culture
Stool culture is positive mostly from the second to third week and also in carriers, so it has limited sensitivity and cannot prove current disease.
D. Bone marrow culture (correct)
Correct. Bone marrow culture has the highest yield after prior antibiotic use and in the later course of illness.

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 Microbiology question

What is the answer to this Microbiology question?

The best answer is D, Bone marrow culture. In enteric fever the bacilli are in the reticuloendothelial system, so bone marrow culture remains positive in most patients even after several days of antibiotics and late in the illness, when blood culture sensitivity has fallen.

Why is "A single Widal test at this visit" not the answer?

A single titre cannot be interpreted in an endemic area because of background antibodies; a rising titre on paired sera is needed, and this is a serological test, not a confirmation of the organism.

Why is "Urine culture" not the answer?

Urine culture is positive only in a minority of patients in the later weeks and is much less sensitive than marrow culture.

Why is "Repeat stool culture" not the answer?

Stool culture is positive mostly from the second to third week and also in carriers, so it has limited sensitivity and cannot prove current disease.

What concept does this question test?

It tests Bacteriology: Salmonella Typhi in Microbiology. Blood culture is the best test in the first week, while stool and urine cultures become positive only in the second and third weeks and are less sensitive. A single Widal titre in an endemic area has poor specificity.

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 Bacteriology: Salmonella Typhi?

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.