Mycology: Pythium insidiosum: 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 · Mycology: Pythium insidiosum

Sample item

A farmer in a waterlogged rice-growing region has a chronic non-healing corneal ulcer after exposure to stagnant water. Scrapings show broad, sparsely septate, hyphae-like filaments, but the ulcer progresses despite natamycin and amphotericin B. What explains the failure of antifungal drugs?

AThe organism forms spores that are impermeable to all drugsBThe organism is an oomycete whose membrane lacks ergosterolCThe organism is a bacterium that has no cell membraneDThe organism is a virus that replicates inside corneal cells

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

The answer and the reasoning

B. The organism is an oomycete whose membrane lacks ergosterol

Pythium insidiosum is an aquatic oomycete, not a true fungus, and causes pythiosis of the cornea, skin and blood vessels. Its membrane has little or no ergosterol, which is the target of polyenes and azoles, so these drugs are ineffective. Treatment relies on surgical excision, and antibacterial agents such as linezolid or azithromycin have been tried.

Option by option

A. The organism forms spores that are impermeable to all drugs
Spores are not the explanation, and the problem is a missing drug target.
B. The organism is an oomycete whose membrane lacks ergosterol (correct)
The absence of ergosterol removes the target of polyenes and azoles.
C. The organism is a bacterium that has no cell membrane
Pythium is a eukaryote with a cell membrane and is not a bacterium.
D. The organism is a virus that replicates inside corneal cells
Pythium is a eukaryotic oomycete and is not an obligate intracellular virus.

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 B, The organism is an oomycete whose membrane lacks ergosterol. Pythium insidiosum is an aquatic oomycete, not a true fungus, and causes pythiosis of the cornea, skin and blood vessels.

Why is "The organism forms spores that are impermeable to all drugs" not the answer?

Spores are not the explanation, and the problem is a missing drug target.

Why is "The organism is a bacterium that has no cell membrane" not the answer?

Pythium is a eukaryote with a cell membrane and is not a bacterium.

Why is "The organism is a virus that replicates inside corneal cells" not the answer?

Pythium is a eukaryotic oomycete and is not an obligate intracellular virus.

What concept does this question test?

It tests Mycology: Pythium insidiosum in Microbiology. Its membrane has little or no ergosterol, which is the target of polyenes and azoles, so these drugs are ineffective. Treatment relies on surgical excision, and antibacterial agents such as linezolid or azithromycin have been tried.

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 Mycology: Pythium insidiosum?

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.