This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System, Subject name: Physiology
[{'content': 'The first urge to void is felt at: \nSelect and explain. \nA. Bladder volume of 400 mL, B. Phase Ia of cystometrogram, C. Phase Ib of cystometrogram, D. Phase II of cystometrogram', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Cystometry refers to the process of studying the relationship between the intravesical volume and pressure, the Cystometrogram refers to a graphical record of this relationship. Normal cystometrogram shows three phases of filling: Phase Ia: It is the initial phase of filling in which pressure rises from 0 to 10 cm H2O, when about 50 mL of fluid is collected in the bladder Phase Ib: It is the phase of plateau which lasts till the bladder volume is 400 mL. The first urge to void is felt at a bladder volume of about 150 mL, and a marked sense of fullness at about 400mL. The flatness of segment Ib is a manifestation of the law of Laplace. Phase II: This phase stas beyond 400 mL volume when the pressure begins to rise markedly, triggering the micturition reflex', 'role': 'assistant'}]