In a patient with severe central diabetes insipidus caused by a lack of anti­diuretic hormone secretion, which part of the tubule would have the lowest tubular fluid osmolarity?

In the absence of ADH, the late distal tubule and CD are not permeable to water. Therefore, the  tubular fluid, which is already dilute when it leaves the loop of Henle (about 100 mOsm/L), becomes  further diluted as it flows through the late distal tubule and CD as electrolytes are reabsorbed. Therefore, the final urine osmolarity in the complete absence of ADH is less than 100  mOsm/L.