Assertion (A) is correct: Milk fever (parturient paresis) occurs commonly within 48 hours of parturition in high-producing dairy cows. Reason (R) is also factually correct: copious milk secretion creates a huge demand for blood glucose for lactose synthesis (which can cause ketosis). However, Milk Fever is pathophysiologically caused by acute hypocalcemia (sudden loss of blood calcium into colostrum), not glucose drain. Thus, (R) is NOT the correct explanation of (A).

Beta-endorphin is derived from the post-translational processing of the precursor polyprotein Pro-opiomelanocortin (POMC), which is synthesized predominantly by the corticotropic cells in both the anterior lobe and the intermediate lobe of the adenohypophysis (pituitary gland).

17β-estradiol (E2) is the primary, most potent, and most biologically active circulating estrogen during the non-pregnant reproductive years, exhibiting an estrogenic potency roughly 10-fold higher than estrone (E1) and 80-fold higher than estriol (E3).

Rennin (chymosin, with double 'n') is a milk-coagulating proteolytic aspartic endopeptidase produced by chief cells of the gastric mucosa in the abomasum/stomach of young ruminants. Note that Renin (with single 'n') is a hormone/enzyme synthesized by juxtaglomerular cells in the kidney.