A 22-year-old man presents with a 4-day history of fever and productive cough with rust-colored sputum. Sputum Gram stain reveals gram-positive diplococci in pairs. He is started on azithromycin, a macrolide that inhibits bacterial protein synthesis by blocking translocation at the large ribosomal subunit. Over the following 5 days his fever resolves and his cough improves; he develops no drug-related systemic toxicity. His mother asks the physician why this drug halts bacterial protein synthesis without harming her son's cells.
Which of the following best explains the selective inhibition of bacterial protein synthesis by azithromycin?