I think Guyton is a bit better, but much bigger. If you’re going to read the whole thing, Costanzo is better. Otherwise, go with Guyton if you’re using just 1 chapter. Renal Physiology is not an area to skimp out on. Definitely learn as much as you can including NH4+. It has a significance when calculating urine anion gap and which is important for non-anion gap metabolic acidosis. Also what the kidneys do for H+/HCO3 excess, contraction alkalosis, and calcium/magnesium stuff are reletively new and important as well as hormones like PTH, Aldosterone, and ADH. You should have Na/K/Cl and aldosterone stuff done in undergrad, but there’s some deeper stuff. Also review the Channelopathies (RTAs). Outside of electrolytes/pH balance, there’s tons of renal Pathology I wouldn’t touch now, but you can understand what an Acute Kidney injury is and the various types. The stages to AKI recovery shows up a lot in UWorld too. Outline the major chemicals used to monitor renal injury and what each represents. Also be able to define renal clearance and know the formulas off the top of your head. Some helpful things for wards are when to order FeNa vs FeUrea