A infant had 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 revolves around the identification of a gram-positive coccus causing invasive disease in an infant, with a focus on susceptibility testing. This involves understanding the characteristics of gram-positive cocci and the specific agents used for their identification and susceptibility testing.

**Why the Correct Answer is Right**
The suspected pathogen is likely to be *Staphylococcus aureus*, which is known for its ability to cause severe infections, including bacteremia and pneumonia. The haemolytic colonies on blood agar are consistent with this organism. The screening test for identification often involves the use of a bile solubility test, which is specific for *Staphylococcus aureus*. This test takes advantage of the fact that *S. aureus* can hydrolyze the bile salts, resulting in a clear zone around the colonies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the description does not match the characteristics of *Staphylococcus aureus*. The agent used for susceptibility testing of this organism is not listed in this option.
**Option B:** This option is incorrect because the description does not match the characteristics of *Staphylococcus aureus*. The agent used for susceptibility testing of this organism is not listed in this option.
**Option D:** This option is incorrect because the description does not match the characteristics of *Staphylococcus aureus*. The agent used for susceptibility testing of this organism is not listed in this option.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that *Staphylococcus aureus* is a common cause of invasive disease in infants, particularly those with underlying medical conditions or those who are immunocompromised. The use of a bile solubility test is a critical step in identifying this organism.

**Correct Answer:** C. Oxacillin.

✓ Correct Answer: C. Optochin