**Question:** In a patient of orbital cellulitis, microorganism on culture shows greenish colonies and optochin sensitivity. The most likely organism is

A. Streptococcus pneumoniae
B. Staphylococcus aureus
C. Haemophilus influenzae
D. Escherichia coli

**Core Concept:**

Orbital cellulitis is a severe form of cellulitis involving the soft tissues of the orbit, which is a potential space within the skull surrounding the eye. Culture and sensitivity play a crucial role in determining the causative organism and its susceptibility to antibiotics, guiding the appropriate treatment.

**Why the Correct Answer is Right:**

The correct answer is Streptococcus pneumoniae (A). Streptococcus pneumoniae is a gram-positive diplococcus bacterium that commonly causes infections in the respiratory tract, meningitis, and sepsis. It exhibits optochin sensitivity, which is a characteristic feature of the organism. Optochin discs contain bile salts which inhibit the growth of Haemophilus influenzae but not Streptococcus pneumoniae.

**Why Each Wrong Option is Incorrect:**

Option B (Staphylococcus aureus) is incorrect because staphylococci are usually resistant to optochin due to the production of bile salt hydrolase enzyme, which inactivates the bile salts in the optochin disc, allowing the organism to grow.

Option C (Haemophilus influenzae) is incorrect because Haemophilus influenzae is typically resistant to optochin due to the enzyme bile salt hydrolase it produces.

Option D (Escherichia coli) is incorrect because Escherichia coli is a gram-negative bacterium, which does not exhibit optochin sensitivity.

**Clinical Pearl:**

Recognizing optochin sensitivity on culture and sensitivity report is crucial in guiding the choice of antibiotics for treating patients with orbital cellulitis. Optochin-sensitive organisms, like Streptococcus pneumoniae, should be treated with beta-lactam antibiotics like amoxicillin or penicillin, while optochin-resistant organisms like Haemophilus influenzae require treatment with macrolides or quinolones.