This sentence comes from Li Sao Notes written by Jia Dao, a poet in the Tang Dynasty. In this poem, there is indeed the phenomenon of object omission. According to the grammatical rules of modern Chinese, the complete sentence should be "I" asked the boy "the boy" my teacher, "and he replied," I went to collect medicine. "
However, in this poem, the expressions of "I" and "lad" are omitted, which makes the poem more concise and implicit, leaving more imagination space for readers. This kind of ellipsis is common in ancient poetry, mainly for the pursuit of rhythm and conciseness of poetry.