**Question:** All the strains of A.A. Comitans are sensitive to:

**Core Concept:** A.A. Comitans is a bacterium, specifically a type of Streptococcus bacteria. Antibiotic sensitivity testing determines which antibiotics a specific strain of a bacterium is susceptible to.

**Why the Correct Answer is Right:** All the strains of A.A. Comitans are sensitive to Penicillin (option C). Penicillin is a beta-lactam antibiotic that inhibits bacterial cell wall synthesis, ultimately leading to bacterial cell death.

**Why Each Wrong Option is Incorrect:**

A. Chloramphenicol (option A) is a bacteriostatic antibiotic that inhibits protein synthesis in bacteria. However, it is not effective against Gram-positive bacteria like A.A. Comitans.

B. Penicillinase-producing bacteria can neutralize Penicillin (option D), making the antibiotic ineffective. In this case, the bacterium produces Penicillinase, an enzyme that breaks down Penicillin, preventing it from inhibiting cell wall synthesis.

C. Gentamicin (option B) is an aminoglycoside antibiotic that targets bacterial protein synthesis. However, it is not specific for Gram-positive bacteria and might not be effective against A.A. Comitans.

**Clinical Pearl:** It is crucial for clinicians to consider antibiotic sensitivity testing results when selecting an appropriate antibiotic therapy for bacterial infections. Inappropriate antibiotic selection may lead to treatment failure and increase the risk of antibiotic resistance development.

**Correct Answer:** C. Gentamicin (1.2 µg/mL)

**Explanation:**
Gentamicin is an aminoglycoside antibiotic that targets bacterial protein synthesis by binding to the 30S ribosomal subunit, inhibiting peptide bond formation. However, it is not specific for Gram-positive bacteria like A.A. Comitans and may not be as effective as Penicillin.

**Core Concept:** Antibiotic sensitivity testing is crucial for determining a suitable antibiotic for treating bacterial infections. Using an antibiotic that is not effective against the causative agent can lead to treatment failure and contribute to antibiotic resistance development.