Give some stream of consciousness and then the answer. Renal pyramids (or malpighian pyramids or Malpighi's pyramids named after Marcello Malpighi, a seventeenth-century anatomist) are cone-shaped tissues of the kidney. In humans, the renal medulla is made up of 10 to 18 of these conical subdivisions. The broad base of each pyramid faces the renal cortex, and its apex, or papilla, points internally towards the pelvis. The pyramids appear striped because they are formed by straight parallel segments of nephrons and collecting ducts. The base of each pyramid originates at the corticomedullary border and the apex terminates in a papilla, which lies within a minor calyx, made of parallel bundles of urine collecting tubules.

Based on this passage, why do renal pyramids have a striped appearance?
To answer this question, we should know that: The pyramids appear striped because they are formed by straight parallel segments of nephrons and collecting ducts.
Therefore, the answer is because they are formed by straight parallel segments of nephrons and collecting ducts.