He had -------------- that made rapid aging in his early years of childhood.

Correct sentence: He had a rare disease that made rapid aging in his early years of childhood.

Explanation: 'Rare' describes a condition that affects a small fraction of the population. The phrase 'rare disease' is the standard collocation to express low prevalence. The indefinite article 'a' is correct because 'rare' begins with a consonant sound.

• Meaning: 'rare' = affecting very few people, which matches the context of an uncommon childhood condition.

• Grammar: use 'a' before 'rare' because it starts with a consonant sound.

• Collocation: 'rare disease' is the conventional expression in English for an uncommon medical condition.

Why the other choices are not correct:

• A unique disease — 'unique' means 'one of a kind' and does not specifically indicate low prevalence; it is less precise than 'rare' for describing an uncommon medical condition.

• an unnaturals disease — this contains a grammatical error ('unnaturals'). If intended as 'an unnatural disease,' that would mean 'not natural' rather than 'uncommon,' so it still does not match the intended meaning.

• a special disease — 'special' is vague and does not clearly convey that the disease is rare; it is not the usual phrase to express low prevalence.