There are two accepted forms for possessive singular nouns ending in s:
Add an apostrophe (') after the existing s at the end of the word: diagnosis'
Add an apostrophe s ('s) after the existing s at the end of the word: diagnosis's
Examples:
The test should support the diagnosis' confirmation.
The test should support the diagnosis's confirmation.
The singular possessive form is diagnosis's.
Quantum is singular, not singular possessive. The singular possessive form is quantum's.
diagnosesThe plural form of diagnosis is diagnoses. It is pronounced dye-egg-no-seas.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive pronouns are "my," "mine," "your," "yours," "his," "her," and "its."
The singular possessive of dish is dish's
The singular possessive is Martin's.
The singular possessive is fish's.
Grave's is the singular possessive.
The singular possessive form of "test" is "test's".
Buzz's is the singular possessive.
The singular possessive is biker's; the plural possessive is bikers'.