**Core Concept**
Rabies immunoglobulin (RIG) is a passive antibody preparation that provides immediate, temporary protection against rabies virus infection. It is administered to individuals exposed to rabies virus to prevent or modify the disease course.

**Why the Correct Answer is Right**
The recommended dose of RIG is 20 IU/kg body weight, not exceeding 40 IU. This dose is administered intramuscularly, preferably in the deltoid region, as close as possible to the wound site. The RIG preparation contains antibodies against rabies virus, which neutralize the virus and prevent its spread to the central nervous system.

**Why Each Wrong Option is Incorrect**
**Option A:** The dose of RIG is not 10 IU/kg, as this may not provide adequate protection against the virus.
**Option B:** The dose of RIG is not 30 IU/kg, as this may not be sufficient to neutralize the virus in all cases.
**Option C:** The dose of RIG is not 50 IU/kg, as this is higher than the recommended dose and may increase the risk of adverse reactions.

**Clinical Pearl / High-Yield Fact**
It is essential to administer RIG as soon as possible after exposure to rabies virus, ideally within 7 days, to ensure maximum efficacy.

**Correct Answer: D. 20 IU/kg.**