A but juxtamedullary nephron is a type of nephron. The "medullary" is referring to the renal medulla, while the "juxta" refers to the relative position of the proximal convoluted tubule. In other words, a "juxtamedullary nephron" is one where the proximal convoluted tubule and its associated loop of Henle occur at a deep position compared to most other nephrons.
This type of nephron is relatively rare, and only comprise 15% of the nephrons in the kidney. However, in most depictions of the nephron, it is a juxtamedullary nephron which is depicted. The greater gradient in the deep medulla make this type of nephron do more "work" than more shallow nephrons, and their vertical nature makes them easier to illustrate.
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