- The enzyme responsible for the conversion of angiotensinogen into angiotensin I in the renin-angiotensin system is Renin.
- Renin is an aspartyl protease secreted primarily by the juxtaglomerular cells of the kidney in response to physiological triggers such as low blood pressure, decreased sodium levels, or sympathetic nervous system activation.

- Angiotensinogen is a plasma protein produced by the liver, which is biologically inactive until cleaved by renin.
- Once renin acts on angiotensinogen, it forms angiotensin I, a decapeptide that is further converted into the potent vasoconstrictor angiotensin II by the angiotensin-converting enzyme (ACE).

To clarify the other options:
- Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II, not angiotensinogen to angiotensin I.
- Aldosterone synthase is involved in aldosterone production in the adrenal cortex, unrelated to the cleavage of angiotensinogen.
- Chymase is an alternative enzyme that can convert angiotensin I to angiotensin II but does not act on angiotensinogen.

Thus, the key initiating enzyme in the renin-angiotensin cascade is renin, making it the correct answer.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 27: Renin-Angiotensin System and Adrenocortical Steroids, Page 316-318