**Core Concept**
The administration of Human anti-rabies immunoglobulin (HRIG) is a critical component of post-exposure prophylaxis (PEP) for rabies, providing immediate, passive immunity against the rabies virus. This is particularly important in cases where the individual has been exposed to a high-risk animal or has not received the full series of rabies vaccinations.

**Why the Correct Answer is Right**
The recommended dose of HRIG for post-exposure prophylaxis is 20 IU/kg, administered intramuscularly as soon as possible after exposure. This dose is sufficient to neutralize the rabies virus and prevent the development of clinical disease. The administration of HRIG is a key step in preventing rabies, particularly in individuals who have not received the full series of rabies vaccinations or have been exposed to a high-risk animal.

**Why Each Wrong Option is Incorrect**
**Option A:** 10 IU/kg is below the recommended dose and may not provide adequate protection against the rabies virus.
**Option C:** 30 IU/kg exceeds the recommended dose and may increase the risk of adverse reactions, such as local irritation or allergic reactions.
**Option D:** 40 IU/kg is significantly higher than the recommended dose and may be associated with an increased risk of adverse reactions.

**Clinical Pearl / High-Yield Fact**
It is essential to note that HRIG should be administered as soon as possible after exposure, ideally within 7 days, and in conjunction with the first dose of rabies vaccine.

**✓ Correct Answer: B. 20 IU/kg**