## Core Concept
Yersinia pestis, the causative agent of plague, is a gram-negative bacterium. Understanding its characteristics is crucial for diagnosis and treatment.

## Why the Correct Answer is Right
The correct statements about Y. pestis include:
- It is a gram-negative bacterium, not gram-positive, which already hints that option a) is incorrect.
- The organism is actually motile at 20-28°C (room temperature) with a characteristic bipolar staining.
- Prophylaxis and treatment often involve antibiotics like doxycycline, ciprofloxacin, or gentamicin; benzyl penicillin is not the first choice for prophylaxis.
- Isolation is typically recommended until 24 hours after the start of effective antibiotic treatment, not necessarily 48 hours.
- Repeated blood cultures can be part of diagnosing septicemic plague.

## Why Each Wrong Option is Incorrect
- **Option A:** Gram-positive. Y. pestis is gram-negative.
- **Option B:** Non-motile. Y. pestis is motile at room temperature.
- **Option C:** Benzyl penicillin is given in prophylaxis. This is not typically recommended for Y. pestis infections.
- **Option D:** Patients are kept isolated till 48 hours of treatment. Isolation is usually until 24 hours after effective antibiotic treatment starts.

## Clinical Pearl / High-Yield Fact
A key point to remember is that Y. pestis is a gram-negative rod and is the causative agent of plague, which can manifest as bubonic, pneumonic, or septicemic plague. Doxycycline is a commonly recommended antibiotic for prophylaxis and treatment.

## Correct Answer: D.