An infant had a high-grade fever and respiratory distress at the time of presentation to the emergency room. The sample collected for blood culture was subsequently positive showing growth of a haemolytic colonies. On Gram staining, these were gram-positive cocci. In the screening test for identification, the suspected pathogen is likely to be susceptible to the following agent

**Core Concept**
The question requires identification of a bacterium based on its Gram staining characteristics and growth pattern on blood agar. This involves understanding the morphology and biochemical properties of different types of bacteria.

**Why the Correct Answer is Right**
The presence of haemolytic colonies on blood agar, along with Gram-positive cocci morphology, is indicative of Staphylococcus aureus. This organism is known for its ability to produce alpha-haemolysis, which results in the partial breakdown of red blood cells, giving a greenish discoloration around the colonies. The correct agent that the suspected pathogen is likely to be susceptible to is Bacitracin, a bacteriocidal antibiotic that is specifically used to differentiate between S. aureus and other coagulase-negative staphylococci. Staphylococcus aureus is resistant to Bacitracin, whereas most other staphylococci are sensitive to it.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the use of Bacitracin sensitivity testing is a key differentiator between S. aureus and other coagulase-negative staphylococci, particularly in cases where the Gram stain and colonial morphology are inconclusive.

**Correct Answer:** None of the options are listed

✓ Correct Answer: C. Optochin