October 25, 2010

USMLE 1 Question 20

Sputum sample was collected from a 60 yr old man suffering from a purulent cough and fever. The man was later found to have lobar pneumonia. The sputum was subjected to a gram staining technique. Gram positive, capsulated, flame shaped diplococci were seen when the stained smear was examined under the microscope. Sputum culture on blood agar revealed dome shape centrally umblicated colonies with evidence of alpha hemolysis.

The organism causing the infection is likely to be

a) Staphylococcus aureus
b) Klebsiella pneumoniae
c) Streptococcus pneumoniae
d) Corynebacterium diptheriae
e) Mycoplasma pneumoniae

The correct answer is C

Streptococcus pneumoniae is a gram-positive flame shaped organism and is seen characteristically as diplococci (i.e. is in pairs) and capsulated. The organism grows on blood agar to form dome shaped centrally umblicated colonies and produces alpha hemolysis of the RBCs.

Staphylococcus aureus is not the correct answer since it is typically seen in clusters under the microscope when gram stained and also its colonies are raised and not dome shaped. It produces beta hemolysis on blood agar rather than alpha hemolysis.

Klebsiella pneumoniae is also unlikely since it is a gram negative rather than gram positive organism. It will be seen under the microscope as rods rather then cocci.

Corynebacterium diphtheriae only causes an upper respiratory tract infection. It does not cause pneumonia so it cannot be brought up in the sputum.

Mycoplasma pneumoniae can cause a pneumonia but is incorrect since it does not take up gram stain and also it does not grow on blood agar.

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