Gardeniasrme

Yes I in this one instance I obtained a paid for medical nexus statement from Dr. Anaise stating my OSA Sleep Apnea was caused by aggravated by my VN long term PTSD and PTSD medications and also My VA VN service connected IHD Agent Orange Heart disease.  He sent the board a 22 page explanation and listing many medical treatise as support for his contentions that won me my BVA appeal.  The C&P examiner and VARO used the excuse I was 30lbs overweight for many years to deny my claim but this did not fly with the BVA.
Even though you may meet the criterial and qualifications for a VA service connection disability you may not be able to receive an important formal doctors nexus opinion or DBQ stating your disability is due to your service or another service connected condition because some doctors and their staffs are lazy, indifferent or hostile to vets receiving VA money.
Here is what has worked for me since 1987 to present day.  I have submitted copies of VA, Army, private medical records and other evidence with my numerous claims and appeals with almost 100 percent success and won those claims/appeals without a doctors nexus statement with only one recent exception where the paid for opinion was also successful.  I have been 100% P&T since 98 and now also with SMC-S.  
IF you contact me I can send you a detailed list of the medical and other evidence that has won my claims and appeals.  What has worked for me is no guarantee it will work for you.
It is a disservice IMHO for anyone to discourage a vet from filing a disability claim just because they cannot get some chicken*** doctor to fill out a DBQ or sign a nexus statement as the vet may still have the other medical and non medical evidence in their files or possession to win. 
I encourage vets to seek opinions of others and do their own research as I have done.
Some vets like to brag about their receiving a VA or private doctors nexus opinion free of charge or it was paid for by their insurance or others and I congratulate them on their good fortune.  I have learned and worked the hard way for my benefits and proud of it.
I learned many years ago to in my case to correctly assume the VARO will deny my claim with a BS statement like "NO EVIDENCE" and then I have to appeal to the BVA or higher U.S. CAVC court which I have done on my own successfully.  This has created jealously on part of some other vets.  Too Bad they can still get happy.
My comment is not legal advice as I am not a lawyer, paralegal or VSO representative.