enlargment of bloob vessle in the lung result from septal defect
"Plethora" is a singular noun, so you say "is a plethora."
Plethora is singular and the plural form is plethoras
(Plethora means an abundant excess as a bountiful excesss such as riches.)"The Thanksgiving feast was a plethora of favorite foods of the family.""There was a plethora of costumes to choose from for the masquerade.""From the plethora of possible names for the project, he chose the most apt."
There was a plethora of food on the table at Thanksgiving.
"Is", since plethora is in singular form
Plethora is a noun.
The noun plethora (a plethora) uses singular verb forms.The word plethora is an uncountable (mass) noun, a type of aggregate noun that refers to an unspecified but large number of elements, e.g. a plethora of choices.
Plethora is singular and the plural form is plethoras.
The plethora of escapes from supposedly secure prisons embarrassed the hapless wardens. The success of his videos inspired a plethora of imitators.
"There was a plethora of items in her desk." "Plethora" is not a verb / action word, so it will not change forms from a present to a past tense.
superabundance, an excess
A "veritable plethora" is a great and genuine excess of something. "Veritable" means, "being truly or very much so," and a plethora is, "an overabundance or excess."