Final answer:
The thickest external layer of the kidneys is the perirenal fat capsule, which is surrounded by the renal fascia and covers the innermost renal capsule.
Step-by-step explanation:
The thickest layer of the external layers that cover the kidneys is the perirenal fat capsule. This layer of adipose tissue provides cushioning and insulation for the kidneys, helping to protect them and keep them in place. Surrounding this fat capsule is the outermost layer called the renal fascia, which is a tough connective tissue layer. The third and innermost layer is the renal capsule, composed of dense, irregular connective tissue. Together, these layers contribute to the robust external anatomy of the kidneys.