In general, concepts are represented in the singular, rather than the plural.
Disorder of lung (disorder), not disorder of lungs
FSNs should not be plural unless the concept necessarily involves multiples.
Unintended plurals might be incorrectly interpreted. An unintended plural is the use of a plural when, in fact, there is only one entity.
Organizational nodes or grouper concepts may be plural.
A concept that necessarily involves multiples should have a plural FSN.
Bilateral atrophy of testes (disorder)
It is advisable to keep track of these exceptions in a separate subset or using a special term type, so that they can be excluded when the singular/plural distinction is important for mapping.