Direct service connection rests on three pillars. The VA calls them the elements; I call them the Big Three. You need all three, and the whole claim is only as strong as the weakest one.

1. A current diagnosis

You must have the condition now, documented by a medical professional. “I used to have it” does not pay. Under 38 CFR §3.303, service connection starts with a disability that exists now, during the life of your claim. That means a diagnosis in your records from a medical professional, not a hunch and not an old note from years ago. Your part is getting seen, getting the condition named and getting it written down.

2. An in-service event, injury, or exposure

Something in service that could have caused the condition, an injury, a stressor, a toxic exposure. Service records, buddy statements, and unit histories build this.

3. A medical nexus

A competent opinion linking the diagnosis to the in-service event, at the “at least as likely as not” standard. This is the pillar the VA attacks most, and the one a strong nexus letter is built to defend.

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.