**Core Concept:** The corpus luteum (CL) is a temporary structure formed after ovulation, which secretes progesterone and estrogen to maintain a stable internal environment for potential implantation. Its regression is a key event in the menstrual cycle.

**Why the Correct Answer is Right:** In the absence of fertilization and implantation, the corpus luteum does not receive the luteotropic effects of human chorionic gonadotropin (hCG) from a fertilized blastocyst. Lack of hCG leads to a decrease in progesterone and estrogen production by the corpus luteum. As a result, enzymes involved in steroidogenesis, such as P450 side-chain cleavage enzyme (P450scc) and 3β-hydroxysteroid dehydrogenase (3β-HSD), are downregulated. This leads to regression of the corpus luteum and subsequent menstruation.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the absence of corpus luteum regression in the presence of fertilization but absence of implantation is not the correct scenario being discussed in the question.

B. This option is incorrect because it assumes that the corpus luteum remains active without hCG stimulation, which would lead to hyperstimulation and hormonal imbalances.

C. This option is incorrect because it assumes that the corpus luteum regression is solely due to decreased blood flow, which is not the primary reason as mentioned above.

D. This option is incorrect as it does not address the absence of fertilization and implantation, which are the key factors leading to corpus luteum regression.

**Clinical Pearl:** The regression of the corpus luteum is an essential process in the menstrual cycle, ensuring that it does not persist unnecessarily and causing hormonal imbalances. This process is regulated by the absence of hCG from a developing blastocyst and the subsequent downregulation of steroidogenic enzymes. Understanding this phenomenon is crucial for understanding menstrual cycle regulation and potential complications in fertility treatments like IVF.