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Is there any general rule to say this must be RNA virus infection and the other one DNA virus infection?

Example of a case: 5 children develop a bright red rash on the face and turns violet after a few days and then disappears. Then maculopapular rash appears on the trunk, buttocks and extremities. It soon fades from the trunk but persists on the thights and forearms. Two children have also had a slight fever and a sore throat, but all were not terribly sick. What is the genetic material of the most likely causative agent?

Right answer is: Single-stranded DNA, since most probably causative agent is erythema infectiosum caused by parvovirus B19.

I do not want to remember all microbes by heart. How can you deduce that this must be a DNA infection and probably single-stranded?

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Regarding 1st q) I will wait in hopes that I'm wrong, but I think the answer is you can't. There is still a lot of work being done just show bacterial vs viral using host markers, let alone breaking down viral. There would also be little clinical significance to knowing, other than epi. I think this is why the red book is now in App form: you ID the pathogen, and then you can know more specifics about it. 2nd question is a totally different question that I would be happy to answer separately (unless someone who can answer 1 in the affirmative can tie them). –  Atl LED Aug 21 '13 at 14:51
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Just as an example of where you can't. Let's say you have a PT with an URI. Most likely cause is viral. Could easily be RSV (-ssRNA), Adeno (dsDNA), HRV(+ssRNA), or any number of others (these were listed for types). Just on the numbers your best guess is HRV, but that could easily be wrong. –  Atl LED Aug 21 '13 at 15:09
    
@AtlLED The second question moved here: biology.stackexchange.com/questions/9934/… You are welcome to answer. I want to know. –  Masi Aug 22 '13 at 20:37
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@AtlLED You're right, there's no way to tell. It's a false dichotomy based on the wording of this exam question. This exam is actually asking us to consider a chain of three questions: What is the disease described; What causes this disease; and How does this causative agent store genetic information. The instructor in this case assumes that if one can answer correctly the last question in this chain then one understands the first two as well. In short, this more a matter of pedagogy than pediatrics. –  Gossar Aug 22 '13 at 22:02
    
@Gossar Off idle curiosity, what is the class you two are taking? Seems like ID, but it's an interesting approach given the questions. –  Atl LED Aug 23 '13 at 3:04
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up vote 1 down vote accepted

If you mean simply by looking at symptoms, then there is no way, especially if in the context of a medical visit. You're better off learning the usual suspects and figuring out the patients risk and likelihood for a given infection. A good example is Hepatitis. Hepatitis B Virus (HBV) and Hepatitis C Virus (HCV) have similar (enough) outward symptoms, but the former is a DNA virus and the latter an RNA virus. In fact, looking at enWikipedia's list of acute causes, it's all over the place!

If you can actually assay for it in the lab, then of course. Although...

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