Find the missing term in the series.
A5BC, A4CE, A3DG, ___________, A1FK

Let us analyze each component of the terms in the series:
1. First letter remains constant: A, A, A, A, A
2. Numerical value decreases by 1: 5, 4, 3, 2, 1
3. Second letter increases by 1 position: B (+1) $\to$ C (+1) $\to$ D (+1) $\to$ E (+1) $\to$ F
4. Third letter increases by 2 positions: C (+2) $\to$ E (+2) $\to$ G (+2) $\to$ I (+2) $\to$ K

Combining these components gives the missing term: A2EI.