To nominalize an adjective that refers to a known noun, the articles el, la, los, and las are used (Unit 11 §59); to nominalize an adjective that does not refer to a known noun, the neuter article lo is used.

But I tell you this has set all London buzzing. The old man is where he likes to be, with a pin-point limelight right on his hairy old head.

“Alphabet of Rabbi Akiba,” 51. 66. 360 f., 364 f., 367

The patient was reviewed in the Accident and Emergency department 90 minutes after FMC.