NBME 23 Osteogenesis imperfecta, type I (NBME Answers)

Retired NBME 23 Answers

nbme23/Block 2/Question#5 (reveal difficulty score)
A female newborn develops respiratory ...
Osteogenesis imperfecta, type I 🔍 / 📺 / 🌳 / 📖
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+14 upvote downvote
submitted by pizzapack(14)

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So once you're down to OI and Rickets think about the pathogenesis of the two diseases and ignore the rest of the stem for a second. Bone is basically two parts. A fibrous "rebar" part and a calcium "cement" part. OI has defective type 1 collagen leading to bones that have no problem mineralizing (cement) but are lacking the rebar. This leads to super brittle bones that break all the time. Like hundreds of fractures.

Vit D deficiency causes an inability to mineralize but has no problem with laying down the collagen or "rebar". As a result you get soft and bendy bones. Sure they can break but the main problem is lack of strength.

From there you pick OI because the kid has 1000 fractures. Rickets wouldn't present that way.

Also bonus buzzword: apparently "wormian appearance" is an important diagnostic clue for OI.

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jaramaiha I believe "wormian" was mentioned by goljan in his audio lectures. +1

veryhungrycaterpillar Very nice explanation. This was my logic exactly, but you put it in words beautifully. +

an1 MULTIPLE fractures in a NEWBORN. OI babies often have lots of fracture during delivery, this is unlikely in rickets from what I know +

an1 okay also for some reason, I recall that in grey's anatomy, Kepner and Avery had a baby with OI yet they were both fine, with no fam history. I know it's not accurate but it was a good way to remember that AD is the most common but not the only way to get the disease, there are exceptions (which NBMe loves...). I bet the person that wrote this had recently binged on grey's before writing this questions smh +1

+9 upvote downvote
submitted by ∗hungrybox(1277)

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TLDR: Physical symptoms >> family history or anything else.

Like the other guy said, I got played hard.

I thought:

• poor prenatal care

• no family history

• bone problem/fractures

Instantly pointed to Rickets.

BUT, in retrospect this is key:

• intercostal retractions (vs. rachitic rosary → costchondral thickenings)

They're basically telling you to rule out Rickets. It seems 100% unfair b/c poor prenatal care seemed to rule in Rickets. The no family history seems to rule out OI.

But I guess what I've learned is, physical symptoms trump ANYTHING ELSE on NBMEs.

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+8 upvote downvote
submitted by ∗seagull(1933)

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The underlying mechanism is usually a problem with connective tissue due to a lack of type I collagen. This occurs in more than 90% of cases due to mutations in the COL1A1 or COL1A2 genes. These genetic problems are often inherited from a person's parents in an autosomal dominant manner or occur via a new mutation.

-Wiki

Basically, good f***ing luck!

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focus Picture of "wormian bones", just for fun: http://www.learningradiology.com/archives2010/COW%20432-Wormian/wormiancorrect.htm +2

+2 upvote downvote
submitted by nukie404(8)

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At first I was thinking aha! Child abuse! But I guess wormian bones are more suggestive of OI, although the no family hx part was rather bleh.

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thotcandy Literally had it on OI until I saw no family hx... Isn't it AUTOSOMAL DOMINANT? +3

faus305 FA 2019 pg. 51. Yes it is autosomal dominant. This question is poorly written but the clues are "wormian bones" and multiple fractures. Pretty sure everyone on this page would've nailed the diagnosis if seeing a real patient instead of using three lines of text with two hints. Questions like this demonstrate how this test became a way of ranking people based off of minutia they know and not their clinical abilities. This is why step is pass/fail in 2 years. +

+2 upvote downvote
submitted by ∗azibird(279)

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Would rickets present in a newborn? This article describing a case series says "There are few other published cases of congenital rickets caused by maternal VDD." So maybe rickets usually presents later in life. Mayo Clinic says it's usually after "an extreme and prolonged vitamin D deficiency."

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2795674/ https://www.mayoclinic.org/diseases-conditions/rickets/symptoms-causes/syc-20351943

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rongloz This was exactly my reasoning of choosing Osteogenesis versus Rickets +1

+1 upvote downvote
submitted by ∗lispectedwumbologist(122)

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How is this the answer if there is no family history of recurrent fractures? I thought osteogenesis imperfecta was autosomal dominant?

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seagull Exactly!! it's an autosomal dominate disease! +10

emcee Autosomal dominant diseases are variably expressive. Still, I think this was a badly written question (should have given us some family history). +

wutuwantbruv Also, FA says that fractures may occur during the birthing process, which is what I believe they were going for. I don't believe these findings would be seen at birth with any of the other choices. +

d_holles Yeah I thought I outsmarted NBME by selecting Rickets bc it said no family history ... guess I got played lol. +9

jean_young2019 Could it be a sporadic cases? Spontaneous Mutation This is a change in a gene that occurs without an obvious cause, in a family where there is no history of the particular gene mutation. OI is inherited as an autosomal dominant trait. Approximately 35% of cases have no family history and are called "sporadic" cases. In sporadic cases, OI is believed to result from a spontaneous new mutation. http://www.oif.org/site/PageServer?pagename=Glossary +6

avocadotoast Amboss says the severe subtypes (types II, III) of OI are usually due to a new (sporadic) mutation in COL1A1 or COL1A2, while patients with the mild forms (types I, IV) typically have a parent with the condition. +

an1 from FA: caused by a variety of gene defects (most commonly COL1A1 and COL1A2). Most common form is autosomal dominant. This doesn't mean there are no other forms, NBME really likes to go for the rear occurrencesoccurances +

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