**Core Concept**
The concept of blood flow per unit of tissue weight is crucial in understanding the physiological needs of different organs. The brain, being the most metabolically active organ, requires a continuous supply of oxygen and nutrients to maintain its high metabolic rate.

**Why the Correct Answer is Right**
The brain has the highest blood flow per unit of tissue weight due to its high metabolic demand. It receives approximately 15-20% of the cardiac output, which is about 750-850 ml/min in a resting adult. This is made possible by the high density of capillaries in the brain, which allows for efficient exchange of oxygen, glucose, and other essential nutrients. The cerebral blood flow is also tightly regulated by various mechanisms, including the autoregulatory response to changes in blood pressure and the release of vasodilatory neurotransmitters such as nitric oxide.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while the heart does receive a significant amount of blood flow, it is not the organ with the highest blood flow per unit of tissue weight. The heart's blood flow is also regulated to meet its specific needs, which are distinct from those of the brain.
**Option B:** This option is incorrect because the liver receives a substantial amount of blood flow, but it is not as metabolically active as the brain. The liver's blood flow is also influenced by factors such as the hepatic portal system and the release of vasodilatory substances.
**Option C:** This option is incorrect because skeletal muscle receives a variable amount of blood flow depending on the level of physical activity. While it does receive a significant amount of blood flow during exercise, it is not the organ with the highest blood flow per unit of tissue weight.

**Clinical Pearl / High-Yield Fact**
The brain's high blood flow is also reflected in its unique vascular anatomy, which includes a high density of capillaries and a lack of significant collateral circulation. This makes the brain particularly vulnerable to ischemic injury in the event of reduced blood flow or increased intracranial pressure.

**Correct Answer: B. Liver.**