The nexus letter is the bridge between your diagnosis and your service. It is also the single most misunderstood document in the claims process. A good one is not about magic phrasing, it is about a credible expert giving a reasoned opinion.

What a strong nexus letter contains

• A qualified author who reviewed your actual records
• A clear statement that the condition is “at least as likely as not” related to service
• A rationale, the medical reasoning that explains why
• Reference to the specific evidence the opinion relies on

The rationale is everything

The VA can reject a bare conclusion. It cannot easily reject a well-reasoned one. “The veteran’s current lumbar degeneration is at least as likely as not related to the documented 2004 parachute injury because…”, that “because” is what gives the letter weight. No rationale, no probative value.

Secondary service connection under 38 CFR §3.310 is developed and adjudicated under M21-1, Part IV, Subpart ii, Chapter 2, Section B (Secondary Service Connection and Aggravation). The VA must obtain a medical opinion addressing whether the claimed condition is "at least as likely as not" caused or aggravated by an already service-connected disability. Under El-Amin v. Shinseki, an examiner who addresses only causation and ignores aggravation has provided an inadequate opinion, because §3.310(b) establishes aggravation as a separate and independent basis for service connection. Under Esteban v. Brown, each secondary condition rates independently and stacks on top of the primary through VA math.