WBR0052

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Author [[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul)]]
Exam Type USMLE Step 1
Main Category Microbiology
Sub Category General Principles, Infectious Disease
Prompt A 6-year-old African American boy is brought to the emergency department for severe abdominal pain. He reports his abdomen is stiff and bloated with diffuse pain that is most severe in the left upper quadrant. The patient has had a previous similar episode in the past. His past medical history is also significant for dactylitis at the age of 18 months. Further questioning reveals that his older sister suffers from similar painful episodes that also require hospitalizations. After supportive measures, this patient is requested to receive a vaccine for an encapsulated, catalase-positive, gram-negative rod. What moiety is used in this vaccine?
Answer A Polyribosylribitol phosphate
Answer A Explanation [[AnswerAExp::The vaccine for Haemophilus influenzae type B is a conjugate vaccine that uses the bacterial capsular polysaccharide polyribosylribitol phosphate (PRP) conjugated with a protein to stimulate long term humoral immunity.]]
Answer B Peptidoglycan
Answer B Explanation [[AnswerBExp::Peptidoglycan composes the membrane of most bacteria. It is not used in the Haemophilus influenzae type B vaccine.]]
Answer C Protein A
Answer C Explanation [[AnswerCExp::Protein A is a virulence factor of S. aureus which binds to the Fc portion of Ig and helps to prevent opsonization and phagocytosis. It is not used in the Haemophilus influenzae type B vaccine.]]
Answer D Lipid A
Answer D Explanation [[AnswerDExp::Lipid A is a part of endotoxin, a lipopolysaccharide which is a highly immunogenic component of the cell wall of gram-negative bacteria. It is not used in the Haemophilus influenzae type B vaccine.]]
Answer E M protein
Answer E Explanation [[AnswerEExp::M protein is a virulence factor of Streptococcus pyogenes that helps prevent phagocytosis. It is not used in the Haemophilus influenzae type B vaccine.]]
Right Answer A
Explanation [[Explanation::The patient in this vignette suffers from sickle cell anemia. Sickle cell anemia is an autosomal recessive hemoglobinopathy that mainly affects patients of African American and Middle Eastern ancestry. Patients with sickle cell anemia suffer from recurrent vaso-occlusive crisis, splenic sequestration crisis, aplastic crisis and hemolytic crisis. All of these clinical scenarios are caused by aggregation of sickled erythrocytes that tend to be brought on by conditions of stress, infection, and dehydration. In this vignette, the patient is suffering from a splenic sequestration crisis. Because of its narrow vessels and function in clearing defective red blood cells, the spleen is frequently affected and is usually infarcted before the end of childhood in individuals suffering from sickle cell anemia. Splenic sequestration crises are acute, painful episodes that cause congestive splenomegaly that eventually lead to autosplenectomy following repeated episodes. During these episodes, the splenic sinusoids and gates would open at the same time resulting in sudden pooling of the blood into the spleen that causes a circulatory defect leading to sudden hypovolemia. The abdomen becomes bloated and very hard on palpation. Splenic sequestration crises are considered an emergency.

Surgical splenectomy may be required in some cases, while others are closely followed-up. Both cases are still characterized by functional asplenia and patients are advised to receive vaccines against encapsulated organisms including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae type B. Haemophilus influenzae type B is an encapsulated, catalase-positive, gram negative rod which can cause severe meningitis in unvaccinated infants and children. The vaccine for this organism is a conjugate vaccine that uses the bacterial capsular polysaccharide polyribosylribitol phosphate (PRP) conjugated with a protein to stimulate long-term humoral immunity.
Educational Objective: The vaccine for this Haemophilus influenzae B is a conjugate vaccine that uses the bacterial capsular polysaccharide polyribosylribitol phosphate conjugated with a protein to stimulate long-term humoral immunity.
References: Vadheim CM, Greenberg DP, Marcy SM, et al. Safety evaluation of PRP-D Haemophilus influenzae type b conjugate vaccine in children immunized at 18 months of age and older: follow-up study of 30,000 children. Pediatr Infect Dis J. 1990;9(8):555-61. First Aid 2014 page 137]]

Approved Yes
Keyword Microbiology, Bacteria, Vaccine, Sickle, Sickle cell, Sickle cell anemia, african american, Sickle cell disease, PRP, hemophilus, infleunzae, autosplenectomy, splenectomy, dactylitis
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