The words known as articles indicate whether a noun is singular or plural. A and an are definite articles. The definite article is 'the'. When referring to something that is singular or has already been mentioned, the definite article "the" is used. In contrast, the article "a" is used with names of nonspecific objects. When a word's first letter is a vowel or has the sound of a vowel, the article "an" is used. As a disease/ ailment is an uncountable noun, no article will be used in the given blank; hence option D is the correct answer.