Pepsinogen is produced by______________ and is activated by__________

A. Tubuloglomerular feedback involves the macula densa sensing changes in sodium chloride concentration and adjusting the glomerular filtration rate (GFR) via the juxtaglomerular apparatus.

B. Renal autoregulation includes mechanisms like the myogenic mechanism and tubuloglomerular feedback that maintain stable GFR despite blood pressure changes. This option is too broad and not specific to the mechanism described.

C. The myogenic mechanism refers to the smooth muscle's tendency to contract when stretched, helping to stabilize GFR by adjusting the diameter of the afferent arterioles. This is the correct answer.

D. Sympathetic control influences renal blood flow and GFR through hormonal and nervous system regulation, but it is not the specific mechanism described in the question.

A. The neural tube and yolk sac have formed: By 8 weeks, the neural tube and yolk sac have formed, but this is not the criterion for defining a fetus.

B. The amniotic cavity has formed: The formation of the amniotic cavity occurs earlier in development and is not the defining feature of a fetus.

C. All of the organ systems are present: By the end of 8 weeks, the basic structures of all major organ systems are present, which is why the embryo is referred to as a fetus from this point onwards.

D. The mesoderm has formed: The formation of the mesoderm occurs earlier and is not the defining factor for fetal status. By 8 weeks, more significant development, including the presence of organ systems, has occurred.