The isolate of S. pneumoniae in the prior question had five penicillin-binding proteins identified, whereas the isolate in the current question only has two.  This indicates that three of the penicillin-binding proteins have been altered in such a way that inhibits the binding of ceftriaxone.  Structural changes in penicillin-binding proteins that prevent ceftriaxone from binding is one mechanism for ceftriaxone resistance.

(Choice A)  Beta-lactamases function to degrade penicillin and cephalosporins.  Production of a beta-lactamase would prevent ceftriaxone from being able to bind to the penicillin-binding proteins.  Because ceftriaxone is so much smaller than the penicillin-binding proteins, unbound ceftriaxone would likely accumulate at one of the electrodes.  This would lead to no bands being found within the electrophoresis area.

(Choice C)  A salvage metabolic pathway can restore metabolic function that would otherwise be inhibited by an antibiotic such as trimethoprim or sulfamethoxazole.

(Choice D)  Transmembranous efflux pumps confer resistance to antibiotics that require entry into the cell to function (e.g., tetracyclines and macrolides).  However, penicillins and cephalosporins function outside of the bacterial cell at the peptidoglycan cell wall.

(Choice E)  Upregulation of protein synthesis can occur to overcome the effect of an antibiotic that binds to and irreversibly inhibits the action of that protein.  However, if this were the mechanism for resistance, five bands would still be identified on the radioautography test.

Educational objective:
A change in the structure of penicillin-binding proteins that prevents cephalosporin binding is one mechanism of bacterial resistance to cephalosporins.