gmataquaguy wrote:
Rheumatic fever is the most serious complication of streptococcal infection, but rheumatic fever is not itself an infection. Rheumatic fever occurs only when, in defending against streptococcal infection, the body's immunological system produces antibodies that also attack the cells of the heart and kidneys.
Which of the following, if true, would contribute most to an explanation of why only streptococcal bacteria give rise to rheumatic fever?
(A) Only certain strains of streptococcal bacteria are associated with rheumatic fever.
(B) The antibodies that the body produces to control a streptococcal infection can attack cells of the mucous membranes
(C) Antibiotics have not proved as effective against streptococcal infections as they have against some other bacterial infections.
(D) Streptococcal bacteria are the only bacteria that are structurally similar to the cells of the body attacked in the rheumatic fever.
(E) Rheumatic fever can be misdiagnosed if it is not preceded by a serious streptococcal infection.
S infection can sometimes lead to R fever.
When antibodies against S infection start attacking the heart and kidneys too, it gives R fever.
We need to explain why ONLY S bacteria causes R fever. What is it about S bacteria infection and the antibodies produced against it that lead to R fever. Why other bacteria do not lead to R fever? What is so distinct about S bacteria?
(A) Only certain strains of streptococcal bacteria are associated with rheumatic fever.
This tells us that R fever happens only in case of some strains of S bacteria. But it doesn't explain why R fever happens only with S bacteria.
(B) The antibodies that the body produces to control a streptococcal infection can attack cells of the mucous membranes
We don't know the relation between mucous membranes and heart and kidney (R fever). This doesn't help explain anything.
(C) Antibiotics have not proved as effective against streptococcal infections as they have against some other bacterial infections.
Doesn't explain why only S infection causes R fever.
(D) Streptococcal bacteria are the only bacteria that are structurally similar to the cells of the body attacked in the rheumatic fever.
S bacteria are the only bacteria that are similar to the cells attacked in R fever (cells of heart and kidney). So now we know that S bacteria are structurally similar to cells of heart and kidney. Hence the antibodies against S bacteria end up attacking heart and kidney too and hence lead to R fever. This is the reason that R fever occurs in case of S bacteria only.
Correct.
(E) Rheumatic fever can be misdiagnosed if it is not preceded by a serious streptococcal infection.
If a serious S infection doesn't happen, R fever can me misdiagnosed as say, viral fever or some other fever. So without a serious previous S infection, doctors may not be able to diagnose R fever correctly. Say if the S infection is mild, doctors may not come to know that it is there and the subsequent fever may not be diagnosed as R fever. But this doesn't explain why only S infection causes R fever.
Answer (D)
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