A 14-Year-Old Patient Presents in the Nephrology Clinic with Fatigue $6 \mathrm{lb}$

A 14-year-old patient presents in the nephrology clinic with fatigue, malaise, anorexia, abdominal pain, and fever. She reports a loss of $6 \mathrm{lb}$ in the past 2 months. Serum gamma globulin and the immunoglobulins $\operatorname{IgG}$ , $\operatorname{Ig} A$ , and $\operatorname{IgM}$ are all elevated. Her serum creatine is $1.4 \mathrm{mg} / \mathrm{dL}$ (normal $0.6-1.2 \mathrm{mg} / \mathrm{dL}$ ) and urinalysis of glucose and protein are $2+$ on a dipstick test, confirmed by laboratory at $8.0 \mathrm{~g} /$ $\mathrm{dL}$ and $0.95 \mathrm{~g} / \mathrm{dL}$ , respectively. A renal biopsy is prepared for light microscopy, and an infiltrate containing lymphocytes, plasma cells, and eosinophils is found among tubules having cells with prominent brush borders. Which one of the following statements correctly pertains to these epithelial cells?

A) Impermeable to water despite presence of $\mathrm{ADH}$
B) The primary site for the reduction of the tubular fluid volume
C) The site of the countercurrent multiplier
D) The site of action of aldosterone
E) Indirectly involved in the release of renin