>  think that disease shows physical symptoms. So, symptoms appear on examination results. Therefore, the doctor can diagnose exatly through a series of test.

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* Correction:     Student's sentence        Corrected sentence        Grammar explanation       Better sentence    

I think that disease shows physical symptoms.

I think diseases show physical symptoms.

Use the plural form of the noun when talking about something that can be true in general. 

So, symptoms appear on examination results.

These symptoms appear on medical examination results.

You may reiterate the same noun that you are referring to, instead of dealing with it a a result. 

Avoid starting your sentence with so in an academic writing.

Therefore, the doctor can diagnose exatly through a series of test.

Therefore, the doctor can diagnose exactly through a series of test.   

Be mindful of the transitional devices that you are using. Would be able is better used as "can" can be stronger.   

Through a series of tests, the doctor would be able to diagnose the patient's exact illness.