Kidney
Step-by-step explanation:
This is because as according to given question,”doctor said that this is due to the malfunctioning of the blood capillaries that help in the filtering of blood."
Here, the malfunctioning of blood capillaries is the issue. As we know that Kidney is the head of urinary system, then only this organ has blood capillaries.
Also, the kidneys are doing the purification of body by removing toxic wastes by urine. If the kidney is malfunctioned, then the remaining urinary bladder, ureter and urethra can do nothing. Their only function is to bring urine to the outside and remove it.
Therefore, the issue is with kidney.