Impact of a C+ grade in a graduate course I'm planning to apply to competitive PhD programs in theoretical physics. Unfortunately, I received a C+ grade in one relevant graduate-level course. I really don't know how I got this grade, because I think I did well in the final examination. The final exam was worth 100 % of the total grade. Now, I received 5 A's, two B+, and two B's in the 10 graduate courses that I took. Unfortunately, my undergraduate degree is unrelated to the field I intend to do a PhD in. I didn't even attend undergraduate courses in calculus. I taught myself math and physics from calculus up to algebraic topology and Quantum field theory. I have also scored high in the subject GRE test. 

I would like to know if these grades would have negative impact on my PhD applications. Should I attempt to do anything about this C+ grade?
One bad mark is unlikely to have much if any effect. Especially if it isn't the most recent. People are looking for indicators of success, so you have to provide plenty of that in your admissions package. Someone may ask you why you got a poor grade, but most will accept a reasonable answer. But your record is what it is and you can't change the past. Apply to the programs that interest you and make your best case.