This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
A 75-year-old male develops a cough that produces blood-tinged sputum. He has a fever of 104degF. Gram-positive cocci in clusters are found in a sputum smear. A chest x-ray shows increased density in the right upper lobe. Of the following penicillins, which is most likely to be ineffective? 
Select and explain. 
A. Oxacillin, B. Cloxacillin, C. Ticarcillin, D. Nafcillin
C: Ticarcillin
Answer: C: Ticarcillin 
Explanation: Ticarcillin resembles carbenicillin and has a high degree of potency against Pseudomonas and Proteusorganisms but is broken down by penicillinase produced by various bacteria, including most staphylococci. Oxacillin, cloxacillin, nafcillin, and dicloxacillin are all resistant to penicillinase and are effective against staphylococci.