**Core Concept**
Iron deficiency anaemia is a condition where the body does not have enough iron to produce haemoglobin, a protein in red blood cells that carries oxygen. The earliest sign of iron deficiency anaemia is a decrease in the body's iron stores, which can be assessed by measuring serum ferritin levels.

**Why the Correct Answer is Right**
Serum ferritin is a protein that stores iron in the body. When iron stores are depleted, serum ferritin levels decrease, indicating iron deficiency. This is the earliest sign of iron deficiency anaemia because it occurs before a decrease in serum iron levels or an increase in total iron binding capacity (TIBC). A decrease in serum ferritin level is a sensitive indicator of iron deficiency and is often used as a diagnostic marker.

**Why Each Wrong Option is Incorrect**
**Option A:** An increase in TIBC (total iron binding capacity) is actually a sign of iron deficiency anaemia, not the earliest sign. In iron deficiency, the body produces more transferrin, a protein that binds to iron, leading to an increase in TIBC.

**Option C:** A decrease in serum iron level is a later sign of iron deficiency anaemia. Serum iron levels decrease when the body's iron stores are depleted, but this occurs after a decrease in serum ferritin levels.

**Option D:** Not all the above options are correct. While a decrease in serum ferritin level is the earliest sign of iron deficiency anaemia, an increase in TIBC and a decrease in serum iron level are also signs of the condition, but they occur later.

**Clinical Pearl / High-Yield Fact**
Remember that serum ferritin level is a sensitive indicator of iron deficiency and is often used as a diagnostic marker. A ferritin level below 15 ng/mL is often considered indicative of iron deficiency anaemia.

**✓ Correct Answer: B. Decrease in serum ferritin level**