Learn Spanish (Latin America): Lo preocupante aquí es tu astigmatismo. - The concerning thing here is your astigmatism.

In Spanish, you can pair the neuter article lo with a masculine singular adjective to talk about an abstract concept. Here, lo turns the adjective preocupante (concerning) into a noun phrase meaning "the concerning thing," "the concerning part," or "what is concerning."

What if the problem was a feminine noun, like tu miopía (your nearsightedness)? Does the adjective after lo need to match it?

No, the adjective would not change. The construction lo + adjective always uses the masculine singular form of the adjective, because lo is a neuter article and doesn't take a gender. Even if you replaced preocupante with a gendered adjective like malo (bad), it would remain lo malo es tu miopía, never lo mala.

Worrying feels like a state or condition. Why does this sentence use es (from ser) instead of está (from estar)?

We use es here because we are defining or identifying what the problem is. The sentence equates one concept (lo preocupante) with another (tu astigmatismo). Whenever you set up an equation like "[Thing A] is [Thing B]," you use ser regardless of whether the things involved sound like temporary feelings or states.

Could I say lo que preocupa instead of ?