Final answer:
Dyspnoea is an early sign of circulatory overload as it indicates the heart is struggling to cope with excess volume, often leading to symptoms like breathlessness.
Step-by-step explanation:
The early sign of circulatory overload among the options provided is dyspnoea (B). Circulatory overload can lead to symptoms like breathlessness on exertion, which is due to the heart struggling to pump the excess volume. This can occur before other symptoms such as ankle oedema, which might develop as the condition progresses. Dyspnoea arises as the circulatory system tries to manage increased fluid volume that it's not equipped to handle efficiently, potentially leading to pulmonary edema if left unchecked.
Renal shutdown (A), while a serious symptom, is more indicative of the late stages of shock or other severe circulatory issues. Ankle oedema (C) can be a sign of progressing circulatory issues but is not typically an early sign. Pain at the site of infusion (D) is generally localized and related more to the infusion process itself rather than being an indicator of circulatory overload.