Question:medium
All of the following statements about Staphylococcus aureus are true, except:
All of the following statements about Staphylococcus aureus are true, except:
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Most staph infections come from the patient's own nasal carriage, not from others.
Updated On: Jun 24, 2026
- The most common source of infection is cross infection from infected people
- About 30 percent of the general population are healthy nasal carriers
- Epidermolysin and toxic shock syndrome toxin are superantigens
- Methicillin resistance is chromosomally mediated
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The Correct Option is A
Solution and Explanation
We need the one wrong line about $Staphylococcus$ $aureus$.
Who carries it: The bug lives quietly in the front of the nose and on skin. About a third of healthy individuals are persistent or intermittent nasal carriers. That makes statement B correct.
Origin of disease: Since carriage is so common, the usual source of a staph infection is the person's own resident flora, an endogenous source. Spread from another infected patient does happen, especially in hospitals, but it is not the most common source overall. So calling cross infection from infected people the commonest source is the inaccurate statement.
Superantigen toxins: TSST 1 is the textbook superantigen, and the epidermolytic (exfoliative) toxins linked to scalded skin syndrome are also counted among the superantigens. Statement C stands.
Basis of methicillin resistance: MRSA owes its resistance to mecA, which sits on the chromosomal SCCmec cassette and encodes an altered penicillin binding protein, PBP2a. That is chromosomal, so statement D is right.
The exception is the claim about cross infection being the leading source.
\[\boxed{\text{Most common source is cross infection from infected people}}\]
Who carries it: The bug lives quietly in the front of the nose and on skin. About a third of healthy individuals are persistent or intermittent nasal carriers. That makes statement B correct.
Origin of disease: Since carriage is so common, the usual source of a staph infection is the person's own resident flora, an endogenous source. Spread from another infected patient does happen, especially in hospitals, but it is not the most common source overall. So calling cross infection from infected people the commonest source is the inaccurate statement.
Superantigen toxins: TSST 1 is the textbook superantigen, and the epidermolytic (exfoliative) toxins linked to scalded skin syndrome are also counted among the superantigens. Statement C stands.
Basis of methicillin resistance: MRSA owes its resistance to mecA, which sits on the chromosomal SCCmec cassette and encodes an altered penicillin binding protein, PBP2a. That is chromosomal, so statement D is right.
The exception is the claim about cross infection being the leading source.
\[\boxed{\text{Most common source is cross infection from infected people}}\]
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