We are in an impartial hearing to get special education for our son. The school social worker testified a tremendous load of lies, distortions and nonsense. She (having set herself up as an armchair expert on everything in the DSM-5) didn't see any outward signs of OCD or anxiety in the child => therefore the highly qualified experts' diagnoses (made after careful work-ups) must be total B.S., and the child's parents must be delusional. I need to describe her (or her statements or her attitude) in my written closing argument. Know-it-all feels too informal for this kind of document. Unhumble isn't a word. Dismissive only covers part of it. I want to get at the arrogance of the ignorant. Well, not exactly ignorant. It's a person who thinks that from a two-week unit in one college course she has now mastered everything there is to know about a complex psychiatric condition. Her knowledge is at the level of a dabbler, but being an anti-snob, she thinks that her down-to-earth intuition and street smarts make her better qualified than the highly trained specialists. (Whom she laughs at.)
It's okay to pull in a character from popular culture who fits this description, perhaps with some sarcasm. Sometimes such an approach is very effective. See this example. If you come up with something really good that is a different part of speech, e.g. a verb, that's okay, I'll try to work with it.
I was asked to give an example sentence. I'll try, but really, I would rather put together a sentence around a particularly effective word or phrase that really gets the idea across. In her conceited ignorance, Ms. X dismisses the child's OCD, on the assumption that if the symptoms are not trotted out in her office, the condition must be a figment of his parents' imagination. (In a separate sentence I'll give a statistic about how long, on average, it takes OCD to get diagnosed, because it can be so well hidden.)
Note 1: the answers suggested to the OP who was looking for an equivalent to a Polish idiom did not have the formal tone I need.
Note 2: Quack, etc., don't work here. If I take my son to see a doctor or therapist, and I think the person is a quack, that's different from the current situation. I would be paying the quack. Here, the social worker is a gatekeeper who is preventing the child from getting needed services and accommodations. She is the queen who gets to decide who gets what, and she can do so in a completely arbitrary way.
Note 3: Charlatan doesn't work because she isn't pretending to be an expert on OCD, etc. She just assumes she has enough knowledge of the subject to say "I did not see any evidence of OCD" => "therefore the parents are blowing a little bit of normal adolescent anxiety out of proportion". Pretentious is a little different from pretender. Pretender doesn't work, for the same reason as for charlatan. She hasn't made unjustified or false claims or statements about her personal status, abilities, or intentions. She just thinks she knows better than the experts (whom she thinks have been manipulated by the horrible, Machiavellian parents).
Note 5: Please don't get too sophisticated on me! If I use a word the hearing officer doesn't know, I'll have to explain it, and then I'm back at square one! Anyway, I'm glad the question got reopened, I'm seeing a number of promising ideas here.