A 7-year-old boy has passed large volumes of urine and craved salty food since infancy, and he is small for his age. His blood pressure is 92/56 mmHg and he has no oedema. Plasma potassium is 2.7 mmol/L and bicarbonate 33 mmol/L. Urinary calcium excretion is high. Urine contains no glucose or protein. After eight hours without fluid, his urine osmolality remains below that of his plasma.
Impaired transport in which one of the following nephron segments best accounts for this boy's salt craving and biochemical results?
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