The question mentions a poem about a doctor afraid of the dentist, but the source passages provided do not contain this poem by Ogden Nash. The poem required to answer this question is not available in the given material.
However, based on general knowledge of Ogden Nash's style in This is Going to Hurt Just a Little Bit:
Nash creates humour through irony — a doctor who treats patients is himself terrified of dental visits. He uses comic exaggeration to describe his fear, employing his trademark irregular rhyme scheme where lines of unequal length end in forced, witty rhymes. His colloquial, conversational tone makes the doctor's cowardly reasoning sound ridiculous yet relatable, entertaining readers through the contrast between the doctor's professional confidence and personal cowardice.
The source passages given cover "The Necklace," Robert Frost's poems, The Invisible Man, and Carl Sandburg's "Fog" — none contain the Ogden Nash poem. In an exam, always answer from the given poem/passage. Key points examiners expect: irony/incongruity, comic exaggeration, unusual rhyme scheme, and relatable humour. If the poem extract is printed in your exam paper, quote specific lines to support each point.