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: Medicine
Acute pyelonephritis and uncomplicated UTI may be differentiated by all of the following, except: 
Select and explain. 
A. WBC cast, B. Concentrating defect, C. Organisms in excess of 100,000 cfu/ml, D. Antibody to Tomm Horsfall protein
C: Organisms in excess of 100,000 cfu/ml
Answer: C: Organisms in excess of 100,000 cfu/ml 
Explanation: Although acute pyelonephritis may be associated with organisms in excess of 100,000CFU/ml, this test cannot be used to localize the infection to the kidneys as such high counts are also seen in uncomplicated UTIs. All other tests mentioned are used to localize the infections to the kidney. Ref: Harrison's Principles of Internal Medicine, 16th Edition, Page 1717; Diagnosis of Genitourinary Disease By Resnick and Older, 2nd Edition, Pages 275-77