**Question:** Quarantine period for yellow fever is:

A. 10 days
B. 21 days
C. 7 days
D. Lifelong

**Core Concept:**
Yellow fever is a viral hemorrhagic fever caused by the yellow fever virus. The primary mode of transmission is through the bite of an infected Haemagogus or Aedes mosquito. The World Health Organization (WHO) recommends certain interventions to control the spread of yellow fever, one of which is quarantine.

**Why the Correct Answer is Right:**
The correct answer is 21 days (Option B) as it is the recommended quarantine period for individuals exposed to yellow fever virus but not infected. This period allows for the incubation period of the virus, which typically ranges from 3 to 6 days, and also covers the period of viremia, during which the virus is detectable in the bloodstream.

**Why Each Wrong Option is Incorrect:**
Option A (10 days) is too short to ensure the virus has been eliminated from the exposed individual. Option C (7 days) is also too short, as the viremia period is typically 3-6 days. Option D (Lifelong) is not practical and does not address the specific risk of exposure to the virus.

**Clinical Pearl:**
In the context of yellow fever outbreaks, quarantine is a crucial measure to prevent the spread of the virus. In addition to quarantine periods, other preventive measures include vaccination, vector control, and public health education.

**Correct Answer:** 21 days (Option B)