Discussion of Keyhole sign
- Sonographic identification of hydronephrosis in a male fetus or infant should prompt investigation for the presence of posterior urethral valves.
- The thick-walled bladder and dilated posterior urethra related to obstruction by the valves, as seen at US, may resemble a keyhole.
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 References for Keyhole sign
- Raymond B. Dyer, Michael Y. Chen, and Ronald J. Zagoria. Classic Signs in Uroradiology. RadioGraphics 2004 24: S247-280S.