This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Which of the following is the investigation of choice for Reversible Myocardial Ischemia? 
Select and explain. 
A. MUGA scan, B. Resting ECHO, C. Thallium scan, D. Coronary angiography
C: Thallium scan
Answer: C: Thallium scan 
Explanation: Thallium 201 is a photon-emitting substance with biologic propeies similar to potassium. It can concentrate inside cells that function normally. Regions that are ischaemic or dead do not concentrate thallium and appear as defects on the scan. Reversible ischemia, such as that induced by exercise, is marked by defects seen at peak exercise that later fill on the delayed scans (reversible defects). In contrast, dead or infarcted regions appear as defects in both exercise and delayed scans (fixed defects). Ref: Medicine By Mark C Fishman, 5th Edition, Page 12; Harrison's Principles of Internal Medicine, 16th Edition, Pages 1324-26