Gram-positive cocci in clusters is consistent with staphylococcus species, most likely staph aureus. Nafcillin is a penicillinase-resistant penicillin and would be appropriate to use here. Vancomycin might be selected if the staph was resistant to methicillin (i.e. methicillin-resistant staph aureus or MRSA). Gentamicin is used for gram-negative, not gram-positive, bacteria. Amphotericin is an antifungal, not antibacterial, medication and is not indicated for this patient.