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Retired NBME 24 Answers

nbme24/Block 3/Question#40 (reveal difficulty score)
A 2-year-old boy is brought to the emergency ...
Polysaccharide protein conjugate vaccine ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
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submitted by โˆ—suckitnbme(239)
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There are two types of pneumococcal vaccines for Strep pneumoniae.

The pneumococcal conjugate vaccine (PCV13) is given to children under 5. It is also given to adults age >=65 who are immunocompromised (PPSV23 is also given afterwards).

The polysaccharide vaccine PPSV23 is given to adults, the indications for which I won't go into.

Since this patient is 2-years old he should have received the conjugate vaccine per CDC guidelines.

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submitted by โˆ—m-ice(370)
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This boy has meningitis caused by Strep pneumoniae, the most common cause of infectious meningitis in general. The vaccine for Strep pneumo is a polysaccharide protein conjugate vaccine. The other major bacteria with a vaccine like this is H. influenzae.

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usmile1  also the meningococcal vaccine! +3

so I thought in kids/elderly you give the diptheria-toxoid conjugate vaccine and adults you give the polysacc conjugate. why does this kid get the polysacc conjugate?


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submitted by theunscrambler(4)
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PCV13 for strep pneumo is polysaccharide conjugated to diptheria toxin-like protein (FA20-105). This is needed for a T-cell dependent response which allows for class switching and production of IgG Ab. This is a much more robust immune response. This provides memory and is why the vaccine is given at an early date.

On the contrary, antigens lacking a protein (such as the capsular polysaccharide) such as PPSV23 vaccine are not presented to T-cells, and therefore have a much weaker response + do not class switch.

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submitted by โˆ—mari17(2)
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Pneumococcal conjugate vaccines are strongly immunogenic in infancy due to both B and T cell recruitment. They provide higher, longer-lasting antibody titers relative to pneumococcal polysaccharide vaccines. The pneumococcal polysaccharide vaccine is poorly immunogenic in infants due to their relatively immature humoral antibody response.

Source: UWORLD

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