SMC-O (38 USC 1114(o)) covers highest anatomical loss: two or more distinct SMC-(L)–(N) conditions, loss of both arms at shoulder, or severe deafness + severe blindness. Paraplegia with loss of sphincter control may also qualify. 38 CFR § 3.350 defines the criteria.
Conditions must be separate and distinct. Multiple distinct medical sources (e.g. one doctor for hearing, another for vision). Avoid pyramiding.