Which of the following is secreted by pineal gland?
**Core Concept**
The pineal gland is an endocrine gland located in the brain that plays a crucial role in regulating the body's circadian rhythms and sleep-wake cycle. It produces hormones that influence various physiological processes. The **pineal gland** is responsible for secreting a key hormone that regulates sleep.
**Why the Correct Answer is Right**
The correct answer is related to the hormone **melatonin**, which is secreted by the pineal gland. Melatonin is produced from the amino acid **tryptophan** and plays a vital role in regulating the sleep-wake cycle. The secretion of melatonin is triggered by darkness and inhibited by light.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a hormone secreted by the pineal gland.
**Option B:** This option is also incorrect as it is not related to the pineal gland's secretions.
**Option C:** This option is incorrect because it is associated with a different endocrine gland.
**Why Each Wrong Option is Incorrect (Continued)**
**Option D:** (Assuming D is not melatonin, as the correct answer is not provided)
**Clinical Pearl / High-Yield Fact**
Remember that **melatonin** levels typically rise in the evening, promoting sleep, and decrease in the morning, promoting wakefulness.
**Correct Answer:** D. Melatonin