The disease-causing microorganism among these is :

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Correct — B, Pathogens. The trick to this question is noticing that one of the four options is not the name of a group of organisms at all. Archaea, bacteria and protozoa are taxonomic groups, defined by cell structure and evolutionary descent; membership in them says nothing about whether an organism causes disease. 'Pathogen' is a functional label: it means, by definition, a microorganism that causes disease — this is the term school science introduces for exactly this purpose (NCERT's chapter on microorganisms states that the microorganisms which cause diseases are called pathogens). So of the four options, only (b) is disease-causing one hundred per cent of the time, because that is what the word means. Every other option is disease-causing only for a minority of its members. The reasoning is definitional, not statistical, and that is what makes the answer clean. Test it against the alternatives. Most bacteria are harmless or positively useful: Rhizobium fixes atmospheric nitrogen in the root nodules of pulses, Lactobacillus turns milk into curd, gut bacteria help synthesise vitamin K, and decomposer bacteria return nutrients to the soil. Most protozoa are free-living inhabitants of soil and water — Amoeba and Paramecium harm nobody. And if 'bacteria' were the answer, every viral disease (polio, dengue, influenza, COVID-19) and every fungal disease (ringworm, athlete's foot) would have to be excluded from the category of disease-causing microorganisms, which is plainly wrong. Only the word 'pathogen' covers all of them.

Why the other options are wrong — (a) The most clearly wrong option in the set. Archaea are single-celled prokaryotes forming a separate domain of life, recognised as distinct from bacteria by Carl Woese in 1977; their cell walls lack peptidoglycan and their membrane lipids are ether-linked, both differences from bacteria. They are best known as extremophiles — methanogens in marshes, paddy fields and the guts of ruminants; halophiles in salt pans; thermoacidophiles in hot springs and volcanic vents. No archaeon has been conclusively established as the causative agent of a human disease, so as a group they are the opposite of a natural answer to a question about disease-causing microbes.  (c) The most tempting wrong answer, because the diseases a student can name most readily — tuberculosis (Mycobacterium tuberculosis), cholera (Vibrio cholerae), typhoid (Salmonella Typhi), tetanus, plague — are bacterial. But 'bacteria' names a domain of prokaryotic organisms defined by their cell structure, and the great majority of its members are harmless or beneficial. A group whose members merely include some pathogens is not itself 'the disease-causing microorganism'; the disease-causing ones within it are precisely the ones we call pathogenic bacteria.  (d) Protozoa are single-celled eukaryotes, most of them free-living in soil, fresh water and the sea — Amoeba, Paramecium, Euglena. A minority are pathogens and they cause some of India's most serious diseases: Plasmodium causes malaria, Entamoeba histolytica causes amoebic dysentery, Leishmania donovani causes kala-azar. Once again the group is defined by its biology, not by causing disease, so it fails for the same reason as bacteria.

Concept: Microorganisms are organisms too small to be seen without a microscope, and they fall into several quite different groups — bacteria and archaea (prokaryotes), protozoa and many algae and fungi (eukaryotes), with viruses treated separately because they are not cellular and can reproduce only inside a host cell. Cutting across this classification is a functional question: what does the organism do to a host? Those that cause disease are called pathogens, and they occur in almost every group — bacteria, viruses, fungi and protozoa all have pathogenic members. The vast majority of microorganisms, however, are harmless or actively useful: they fix nitrogen, ferment food, produce antibiotics, and decompose dead matter to recycle nutrients.

State PCS 2024 Q64 — Science & Technology PYQ with Answer

State PCS 2024 Q64 — Science & Technology PYQ with Answer