What is the first thing an integration architect should validate if a callout from a...

When an integration initiated from the client-side (the browser) fails, the architect must first look at the browser's security policies. In Salesforce, Lightning Web Components are subject to the Lightning Component framework's Content Security Policy (CSP).

CSP is a security layer that prevents cross-site scripting (XSS) and other code injection attacks by restricting which domains the browser is allowed to communicate with. If an LWC attempts to make a fetch() call to an external REST endpoint, the browser will block the request unless that specific domain is whitelisted in CSP Trusted Sites.

Option B (Remote Site Settings) is a common distractor; these settings are strictly for server-side Apex callouts and have no effect on client-side JavaScript requests. Option A (CORS) is also a browser security mechanism, but it must be configured on the external server to allow Salesforce to access its resources. While CORS is necessary, the first thing to validate within the Salesforce environment for a failing LWC callout is the CSP Trusted Site entry. Without this whitelisting, the request will be terminated by the browser before it even leaves the client, regardless of how the external server is configured.