This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System (Kidney, Bladder) Acid-Base Balance, Subject name: Physiology
[{'content': 'Which of the following does not have a Tm value? \nSelect and explain. \nA. Na+, B. Urea, C. Creatinine, D. Glucose', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Transpo maximum (Tm) or tubular maximum : is the maximum amount of a substance that can be transpoed across the tubule (reabsorbed or secreted) per unit time.E.g., glucose has a Tm value of 320 mg/min. Reabsorption or secretion of most substances is mediated by their respective carrier proteins in the wall of the tubules. Tm implies saturation of the carrier. Since Na+ reabsorption primarily involves the activity of Na+/K+ pump which does not exhibit saturation kinetics, Na+ reabsorption does not have a Tm value.', 'role': 'assistant'}]