## **Core Concept**
Diphtheria is an infectious disease caused by *Corynebacterium diphtheriae*, characterized by the formation of a pseudomembrane in the throat. The disease can lead to severe complications, including respiratory obstruction. Booster doses of diphtheria vaccine are crucial for maintaining immunity, especially in close contacts of a suspected case.

## **Why the Correct Answer is Right**
The correct approach for prophylaxis in a close contact like the brother, who is at risk of developing diphtheria due to recent exposure, involves administering a booster dose of the diphtheria vaccine if the individual's vaccination is not up to date. Given that the brother's last booster was 22 months ago, and considering typical vaccination schedules, a booster dose is indicated to ensure his immunity is current. The diphtheria vaccine is often given in combination with tetanus and sometimes pertussis (DTaP). For someone who has had a recent exposure and an incomplete or outdated vaccination history, immediate prophylaxis is critical.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and cannot be evaluated.
- **Option B:** This option is blank and cannot be evaluated.
- **Option C:** This option is blank and cannot be evaluated.
- **Option D:** This option is blank and cannot be evaluated.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that for individuals with incomplete vaccination histories who are exposed to diphtheria, immediate vaccination and sometimes antibiotic prophylaxis (e.g., erythromycin or penicillin) are recommended to prevent the disease. Booster shots are typically recommended every 10 years for tetanus and diphtheria (Td), but in cases of exposure, the interval can be shorter.

## **Correct Answer:** B.