Raymond J. Johnson Jr. wrote:

This house is 50 years old, but the upstairs bathroom is considerably
newer--probably 8-10 years. Within the last week pressure to the sink
has dropped to almost nothing, both hot and cold. Thinking it might be
some sort of obstruction in the supply lines or faucet, I replaced both,
but no dice. The pressure in the adjacent toilet and shower is fine. Any
ideas?

Unscrew the "aerator" on the spout and clean the screen.
More than likely a clump of "gunk" (technical term)
got deposited there. Could have come from the older
piping in the house or even from the Mains outside.

Jim