Final answer:
The proximal convoluted tubule (PCT) is most susceptible to decreased afferent blood flow and oxygen delivery due to its high metabolic activity.
Step-by-step explanation:
The part of the nephron most susceptible to decreased afferent blood flow and oxygen delivery is the proximal convoluted tubule (PCT). Because the PCT is the most metabolically active part of the nephron, which utilizes a host of cellular mechanisms, such as symporters, antiporters, and ATPase active transporters to maintain homeostasis, it has high energy demands and therefore requires a constant supply of oxygen to fuel these processes. The mechanisms involved, including the arteriolar myogenic mechanism and tubuloglomerular feedback, are essential to autoregulate the blood flow and GFR. Decreased blood flow compromises the function of these mechanisms, ultimately reducing the PCT's ability to reabsorb substances efficiently and maintain homeostasis.