The language in the example above is specific. Any language less specific may not meet the standard that VA will require.
Any physician who is qualified to write such a letter on your behalf may do so. While it is commonly believed that VA physicians aren't allowed to write such letters, that isn't true. VA physicians, as with many civilian physicians, simply don't like to write such letters as they are not skilled at the task, may not have the tools to write the letter at hand and they are often so busy caring for a heavy load of patients this is seen as work that isn't a priority.
It is perfectly acceptable for the veteran to write the letter on behalf of the physician and then ask the physician to sign it. In any setting, whether VA or civilian, the veteran is advised that he or she should not ask a nurse or clerk to perform the task of obtaining a signature for them. These ancillary members of the care team often see themselves as "gatekeepers" to guard and protect the physician from tasks that will only take up more valuable time.
They may believe that "rules" or "law" won't allow the physician to sign such a document and the veteran may be refused access to the doctor. It's best to make a routine appointment, wait until the veteran is face to face with the M.D. and ask that provider directly.
Most physicians will sign such a letter if it is brief, to the point and factual. When writing a nexus letter great care must be given to recording only facts and the doctor's conclusions.
There are physicians available who will perform records reviews and/or Independent Medical Examinations and provide opinions. Often these doctors do a very good job of providing a nexus letter if the veteran isn't otherwise able to obtain one.
The charge for such a letter from an independent physician can vary depending on the extent of the services. The veteran must pay that fee in advance with no guarantee that any award will be won. The Independent Medical Examining doctor does not treat or prescribe medications but only provides services of review and report.
The importance of the nexus letter can't be overstated. In many situations the nexus letter from an expert is the only evidence that will tip the scales in favor of the veteran.
In the C & P Service Clinicians Guide instructions are given to the examiner as to the exact phrasing that should be used, as follows here;
Q: How do I give an opinion for Nexus (relationship to a military incident)?
A: When asked to give an opinion as to whether a condition is related to a specific incident during military service, the opinion should be expressed as follows:
1. “is due to” (100% sure)
2. “more likely than not” (greater than 50%)
3. “at least as likely as not” (equal to or greater than 50%)
4. “not at least as likely as not” (less than 50%)
5. “is not due to” (0%)
https://www.benefits.va.gov/benefits/mstcoordinators.asp