A patient admitted to an ICU is on central venous line for the last one week. He is on ceftazidime and amikacin. After 7 days of antibiotics he develops a spike of fever and his blood culture is positive for gram positive cocci in chains, which are catalase negative. Following this, vancomycin was staed but the culture remained positive for the same organism even after 2 weeks of therapy. The most likely organism causing infection is-

**Core Concept**
The underlying principle being tested involves the identification of a specific type of bacterial infection, particularly one that is resistant to common antibiotic therapies. The patient's condition suggests a case of **bacteremia** caused by a **gram-positive cocci in chains**, which is **catalase negative**, indicating the organism is likely a type of **Streptococcus**.

**Why the Correct Answer is Right**
Given the organism is gram-positive cocci in chains and catalase negative, this points towards **Streptococcal** species. However, the persistence of the organism despite vancomycin therapy suggests a resistance pattern. Among Streptococci, **Enterococci** are known for their intrinsic resistance to many antibiotics, including a notable resistance to vancomycin, which is a critical clue in this scenario.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect if it suggested an organism that is not consistent with the gram-positive cocci in chains, catalase-negative profile, or if it didn't account for vancomycin resistance.
**Option B:** Similarly, if this option does not align with the bacteriological description and the resistance pattern, it would be incorrect.
**Option C:** This option would be incorrect if it proposed an organism that doesn't fit the provided microbiological characteristics or the clinical scenario of vancomycin resistance.
**Option D:** If this option suggested a non-Streptococcal organism or failed to explain the vancomycin resistance, it would be the wrong choice.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **Enterococci** can exhibit resistance to vancomycin, a critical antibiotic used for treating serious gram-positive infections. This resistance pattern is a significant concern in healthcare settings, particularly in cases involving central lines and prolonged antibiotic use.

**Correct Answer:** D. Enterococcus faecium

✓ Correct Answer: C. Enterococcus faecalis