What is this fruit found in a market in Uyuni, Bolivia at the end of December 2019? (orange skin, 4cm, white flesh)

Question

What is this fruit found in a market in Uyuni, Bolivia at the end of December 2019?

Fruit diameter: ~4 cm. Seed: large, spherical, black. Flesh: white. Taste: sweet but not as sweet as a lychee.

Answer 1

I found the name: it appears to be Garcinia humilis, known commonly as achachairú or achach:

Garcinia humilis, known commonly as achachairú or achacha, is a small, prolifically-fruiting tree related to the mangosteen. It grows in the southern part of the Amazon basin in the central area of Bolivia, but has recently been planted on a commercial scale in Burdekin, Australia.

That matches the area, appearance and season.