In which populations is adult T-cell lymphoma most commonly found?

Adult T-cell lymphoma (ATLL) is most commonly associated with infection by the human T-lymphotropic virus type I (HTLV-I). This virus has a particularly high prevalence in specific geographic regions and populations. The populations most affected include individuals from Japan, the Caribbean, and parts of West Africa, as well as those with certain demographic links, such as intravenous drug users in these regions.

This specific association with IV drug users is significant, as HTLV-I is spread through routes such as sexual contact and sharing of contaminated needles, which is common in drug-using populations. Therefore, these regions and populations have a notably higher incidence of adult T-cell lymphoma due to both the prevalence of the virus in the community and risk factors associated with its transmission.

Other populations mentioned do not show the same high rates of ATLL as those described in the correct answer. For example, while certain cancers or conditions may be more prevalent in specific racial or geographic groups elsewhere, they do not align with the epidemiologic data regarding ATLL, which is clearly linked to the aforementioned populations.