The example 33 in the article "After the flop, you have initiative" of the Big Stack Strategy recommends to fold against a donk bet in the river (see http://www.pokerstrategy.com/strategy/bss/1565/4/ for the hand details).

However, a similar hand is presented in question 11 of the quiz "Beginner Quiz (1) - NL Fullring". In this case it is recommended to call the donk bet in the river (see http://www.pokerstrategy.com/quiz/weekly/1553/ for the hand details).

I've calculated the equity for the first example.

Equity Win Tie
BU 30.29% 27.88% 2.40% QQ-22, AKs, K9s+, AhQh, AhJh, QhJh, AhTh, JhTh, Ah9h, Th9h, Ah8h, 9h8h, Ah7h, Ah6h, 7h6h, Ah5h, Ah4h, Ah3h, Ah2h, AKo, K9o+

We need to pay \$10 to win \$30.10, we need 33% equity, so I guess we can call.

On the other hand, the equity for the quiz is the following.

Equity Win Tie
MP1 23.88% 19.40% 4.48% QQ-22, AKs, KQs, AhQh, AhJh, AhTh, Ah9h, AKo
BU 76.12% 71.64% 4.48% AcKc

We need to pay \$3.80 to win \$20, which is 19% equity, so we can call too.

As I'm a beginner, I'm not sure if the hand ranges I've assigned are reasonable. Anyway, the only difference I see between the two hands is that a flush is possible in the first one and not in the second.