The easiest straightforward way is to use slices.MinFunc as documented here

https://cs.opensource.google/go/go/+/go1.23.3:src/slices/sort.go;l=79

In your case:

firstYoungest := slices.MinFunc(someSlice, func(a, b patient) int {
    return cmp.Compare(a.bodyTemp, b.bodyTemp)
})