In Country X, diagnosis of
Why this is right
This ends up being a Reporting vs. Incidence type of resolution. Since you wouldn't report that you had a disease unless you were diagnosed with that disease, this answer leads us to think that in country P, the only people who would be reporting that they have contracted P would be people with the most severe cases of the disease. That would explain why this group has such a high rate of death. This also explains why the reports of the overall incidence of disease P are lower than those of other countries: P is only diagnosing / reporting the super severe cases, so it's very much under-counting how much its residents have disease P. If the U.S. only counted covid cases based on who showed up to the hospital needing help, they would be massively undercounting the number of people whose covid is too mild to require hospital treatment -- and the mortality rate of covid would seem really high, because lots of people hospitalized for it end up dying from it. Is this answer providing a distinction about Country X? Not explicitly, but often on Paradox they'll be asking us to make a common sense inference that something kinda weird (like "only diagnose the most severe cases") is more of an outlier trait than a typical trait. Additionally, the best available answer might just give us a possible story to tell, even if doesn't do much to guarantee that it is true. This answer doesn't guarantee us that "unlike other countries, X only reports the worst cases of P", but it at least gives us a story to tell for why X's death rate for disease P is worse than others'.
Skill tested: Paradox · how this choice captures the argument's function is the move to repeat next time.