An 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 α- 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**
α-haemolytic colonies on blood agar are typically associated with partial breakdown of red blood cells, whereas β-haemolytic colonies indicate complete breakdown, and greenish discoloration suggests no haemolysis. Gram-positive cocci are a diverse group of bacteria, but certain species are more likely to exhibit α-haemolysis.

**Why the Correct Answer is Right**
The most common cause of α-hemolytic colonies in a clinical setting is *Streptococcus pneumoniae*. This species is a Gram-positive diplococcus that exhibits partial hemolysis on blood agar. The partial breakdown of red blood cells is due to the action of the enzyme pneumolysin, which is produced by *S. pneumoniae*. The organism is typically susceptible to penicillin, which is a narrow-spectrum β-lactam antibiotic that targets cell wall synthesis in bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because *Staphylococcus aureus*, a Gram-positive coccus, typically exhibits β-haemolysis or no haemolysis on blood agar. Although it can be susceptible to penicillin, the clinical presentation and laboratory findings suggest *S. pneumoniae*.
**Option B:** This option is incorrect because *Enterococcus faecalis*, a Gram-positive coccus, typically exhibits no haemolysis or minimal haemolysis on blood agar. Although it can be susceptible to penicillin, the clinical presentation and laboratory findings suggest *S. pneumoniae*.
**Option C:** This option is incorrect because *Streptococcus pyogenes*, a Gram-positive coccus, typically exhibits β-haemolysis on blood agar. Although it can be susceptible to penicillin, the clinical presentation and laboratory findings suggest *S. pneumoniae*.

**Clinical Pearl / High-Yield Fact**
*Streptococcus pneumoniae* is a common cause of community-acquired pneumonia, meningitis, and otitis media in children. The organism is often susceptible to penicillin, but resistance can occur due to mutations in the penicillin-binding proteins.

**Correct Answer:** C.

✓ Correct Answer: C. Optochin