Question

All of the following statements about Staphylococcus aureus are true, except:

• A. The most common source of infection is cross infection from infected people
• B. About 30 percent of the general population are healthy nasal carriers
• C. Epidermolysin and toxic shock syndrome toxin are superantigens
• D. Methicillin resistance is chromosomally mediated

Hint:

Most staph infections come from the patient's own nasal carriage, not from others.

Answer 1

The Correct Option is A

Step 1: The task is to find the false statement about Staphylococcus aureus.
Step 2: Carriage. The main reservoir of S. aureus is the human nose and skin. Around 30 percent of healthy people carry it in the anterior nares without symptoms. So option B is true.
Step 3: Source of infection. Because so many people are healthy carriers, most staphylococcal infections are endogenous, that is they come from the patient's own carrier flora rather than cross infection from another infected person. The statement that the most common source is cross infection from infected people is therefore incorrect for general staphylococcal disease.
Step 4: Toxins. Toxic shock syndrome toxin 1 is a classic superantigen, and the exfoliative or epidermolytic toxins also behave as superantigens. So option C is acceptable as true.
Step 5: Resistance. Methicillin resistance in MRSA is due to the mecA gene carried on the SCCmec element, which is integrated into the bacterial chromosome. So methicillin resistance is chromosomally mediated, and option D is true.
Conclusion: The exception is option A, since most infections arise from the patient's own nasal or skin carriage, not from cross infection from infected people.