Hematology & oncology Clinical Case / MCQS / Uworld for Usmle step 2 / case7 with answer and explanation and references and Educational objective
A
34-year-old man is brought to the emergency department due to several hours of confusion.
His wife reports that he has had fever. malaise. and cough for the past 2 days.
A year ago, the patient required prolonged hospitalization and extensive
surgery for multiple gunshot wounds to the abdomen. He takes no medications
regularly and has no other medical problems. The patient does not use tobacco,
alcohol, or illicit drugs.
He has no
history of recent travel. Temperature is 40.5 C (104.9 F), blood pressure is 80/50
mm Hg. pulse is 11 0/min, and respirations are 32/min. Mucous membranes are moist
and no cervical lymphadenopathy is present. Dullness to percussion and crackles
over the left lower chest are present. Cardiovascular examination reveals
normal first and second heart sounds and bounding peripheral pulses. The
abdomen has several well-healed surgical scars. Intravenous fluids and
broad-spectrum antibiotics are initiated. The next day, blood cultures show
gram-positive cocci. Which of the following is the most likely underlying
mechanism leading to this patient's clinical presentation?
|
A Complement deficiency . B. Destruction of CD4+ cells . C. Immunoglobulin A deficiency . D. Impaired antibody-facilitated phagocytosis . E. Impaired B cell isotype switching . F. Impaired chemotaxis . G. Impaired oxidative burst . |
Answer : D
Explanation :
This patient likely had a splenectomy during his operation for multiple gunshot wounds to the abdomen. He now has high fever, hypotension, tachypnea, and tachycardia in the setting of bacteremia with gram-positive cocci, suggesting an overwhelming Streptococcus pneumoniae infection.
Encapsulated organisms such as S pneumoniae, Haemophilus influenzae, and Neisseria meningitidis have a polysaccharide exterior that conceals antigenic epitopes and resists innate phagocytosis. Therefore, these pathogens are largely eliminated via the humoral immune response with antibody-mediated phagocytosis (opsonization) and antibody-mediated complement activation. Much of this is dependent on splenic macrophages and the generation of splenic opsonizing antibodies. As such, patients with asplenia are at high risk for fulminant infection with encapsulated organisms. These patients should be immunized with pneumococcal, meningococcal, and H influenzae type B vaccines and take oral antibiotics early in the course of any febrile illness.
(Choice A) Deficiencies of the complement system can be inherited or acquired (eg, systemic lupus erythematosus, antiphospholipid antibody syndrome) and can increase risk of infection with encapsulated bacteria (classic complement pathway). However, such deficiencies are rare, and splenectomy is far more likely to be the cause of this patient's bloodstream infection given his history of multiple abdominal gunshot wounds.
(Choice B) CD4+ lymphocyte cells are destroyed by HIV, and patients are at increased risk of infection with a wide range of pathogenic organisms (including encapsulated bacteria). However, this patient is more likely to have asplenia with defects in antibody production and antibody-mediated phagocytosis
(Choices
C and E) Impaired 8 cell isotype switching is seen in some patients with common
variable immunodeficiency. They have normal 8 cell numbers but significantly reduced
lg subtypes. Patients with common variable immunodeficiency are at risk for
recurrent infections (eg, sinopulmonary, gastrointestinal) and autoimmune disease. Patients with lgA deficiency are often asymptomatic and rapid, fulminant infection is uncommon. This patient is far more likely to have had a splenectomy.
(Choice F) Chemotaxis is impaired in patients with leukocyte adhesion deficiency. Patients with this condition have recurrent bacterial infections, primarily of the skin and mucosa.
(Choice G) Patients with chronic granulomatous disease have impaired oxidative burst and often present with recurrent bacterial or fungal infections due to catalase-producing organisms (eg, Aspergillus nidu/ans, Staphylococcus aureus). Host response against pneumococci (catalase-negative) is not affected by oxidative burst deficiency.
Educational objective:
Patients
with asplenia are at risk for fulminant infection with encapsulated bacteria
(eg, Streptococcus pneumoniae, Haemophilus influenzae) due to deficits
in antibody response and antibody-mediated phagocytosis/complement activation.
0 comments:
Post a Comment