According to Liljeblad, Mordellistena pustulata is supposed to have "Posterior tibiae with three short, oblique, parallel ridges, subapical not included; first segment of posterior tarsi with three, second with two ridges." (or 3,3,2, as I like to express them) "In very small individuals there may be only two ridges and a rudimentary third ridge."

Size: "2-4mm to apices of elytra".
I measure about 2.5mm for this one (in tuck position as imaged). So I guess that's on the "very small" end of the scale.

I think I see the 3,3,2 ridge pattern (remember, don't count the ridge on or just in front of the apex of each segment - often more resembles row of seta extending beyond apex. This beetle as a large subapical one on the tibia), although the last ridge of each segment appears somewhat reduced - nearly "rudimentary".

Comments welcome :)