What is the outermost layer of a kidney?

The outermost layer is a tough connective tissue layer called the renal fascia.

renal capsule membrane

Explanation:

Left: Each kidney is comprised of three layers encapsulated by a fibrous outer layer called the renal capsule membrane. The three kidney layers include the peripheral layer, called the cortex, and the inner layer, called the medulla, which is subdivided into the inner and outer medulla.