**Core Concept**
The Cahill cycle, also known as glucose-alanine cycle, is a metabolic pathway that involves the conversion of muscle glycogen to glucose, which is then transported to the liver for gluconeogenesis. This cycle plays a crucial role in maintaining blood glucose levels during fasting or exercise.

**Why the Correct Answer is Right**
The Cahill cycle involves the pair of organs: liver and muscle. In the liver, the enzyme alanine aminotransferase (ALT) catalyzes the conversion of alanine to pyruvate, which is then used for gluconeogenesis. In the muscle, the enzyme pyruvate dehydrogenase (PDH) catalyzes the conversion of pyruvate to acetyl-CoA, which is then used for energy production. The cycle involves the transport of alanine from muscle to liver, where it is converted back to pyruvate, and then to glucose.

**Why Each Wrong Option is Incorrect**
* **Option A:** Kidney and heart. The kidney is involved in the regulation of electrolytes and acid-base balance, but not directly in the Cahill cycle. The heart is a muscle, but it is not the primary organ involved in this cycle.
* **Option B:** Pancreas and stomach. The pancreas is involved in the regulation of blood glucose levels through the secretion of insulin and glucagon, but it is not directly involved in the Cahill cycle. The stomach is involved in the digestion of food, but it is not directly involved in this metabolic pathway.
* **Option C:** Spleen and lung. The spleen is involved in the filtration of the blood and the storage of red blood cells, but it is not directly involved in the Cahill cycle. The lung is involved in the exchange of gases, but it is not directly involved in this metabolic pathway.

**Clinical Pearl / High-Yield Fact**
The Cahill cycle is an important mechanism for maintaining blood glucose levels during fasting or exercise. It highlights the inter-organ communication and cooperation that occurs in the body to maintain homeostasis.

**Correct Answer: C. Liver and muscle.**