Final answer:
The most reliable sign of overhydration in a dog under fluid therapy for shock is pulmonary edema. Other potential indicators include limb or abdominal swelling, sudden weight increase, and changes in mental status.
Step-by-step explanation:
The most reliable indication that a dog undergoing fluid therapy for shock is overhydrated can be pulmonary edema, which is fluid accumulation in the lungs. This condition leads to breathing difficulties and can be identified by assessing the dog's breathing pattern, listening to the lungs for abnormal sounds, and, if available, through imaging such as X-rays. Other signs of overhydration can include swelling of the limbs or abdomen, a sudden increase in body weight, and in severe cases, alterations of mental status due to electrolyte imbalances. In the case of the dog undergoing fluid therapy for shock, if the dog is overhydrated, its urine output will fall dramatically. This typically means that the dog is not able to excrete the excess fluids and is retaining them.