This patient complained of prolonged vomiting and abdominal pain.

With this history, the ECG is pathognomonic. What is it?

First, there are narrow J-waves that are similar to Osborn waves.  The temperature was normal.  I am not certain, but I believe these are just exaggerated J-waves with nonspecific etiology.

More importantly, there are large U-waves best seen in leads V3 and V4.    You can also see that lead II has no visible T-wave but only a large U-wave.  (If you look at V3 directly above lead II, you see both a T-wave and U-wave.  If you follow that down to lead II below, you see no T-wave corresponding to the T-wave in V3, only a large U-wave.  This is also true for V5 and V6.)  Thus, what appears to be a long QT interval in these leads is really the QU interval.

This is nearly pathognomonic for hypokalemia.  The K was 2.4 mEq/L.  Magnesium was normal.