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?