Not Too Sure

Looking at a patient with swelling at the back of his knee,

Prof: What's your differential?
Registrar a: Baker's cyst....semimembranosus bursa.... synovial sarcoma....popliteal artery aneurysm....etc.

Turn to another registrar,

Prof: Whatelse?
Registrar b: Not Too Sure
Prof: Nitrogen sulphide?
All: ( Huh??)
Prof: N 2 S = nitrogen sulphide

(--')