Final answer:
In malignant disorders, urobilinogen can cause urine to turn dark brown-black when exposed to room air, as it oxidizes upon standing. This is distinct from changes caused by hemoglobinuria or bilirubin, which do not darken urine upon exposure to air.
Step-by-step explanation:
In certain malignant disorders, a urine component that turns urine a dark brown-black color upon exposure to room temperature/room air is d. Urobilinogen. This compound is formed from the breakdown of hemoglobin and is typically a normal component of urine, giving it its characteristic yellow color when converted to urochrome. However, in certain conditions like multiple myeloma and other malignancies, there can be an increased excretion of urobilinogen, which can oxidize and turn the urine dark upon exposure to air. Other conditions that can cause changes in urine color include the presence of hemoglobinuria and excessive bilirubin due to liver disorders, but these do not cause urine to darken on exposure to air.