In a stockpot, combine water, lime leaves, soy sauce, pepper, lemon juice, sugar & lemon grass. Bring to a boil, reduce heat & simmer for 15 minutes. Add cayenne, tofu & mushrooms. Simmer for 5 minutes. Add bok choy & simmer until tender crisp, 2 to 3 minutes. Discard lime leaves. Ladle hot soup into individual serving bowls. Garnish with scallions, cilantro & lime wedges & serve. NOTES: 1. Dried kaffir leaves add a citrus flavour. They can be purchased dried at Asian grocery stores. Or substitute 1 ts grated lime peel. 2. Lemon grass, also available from Asian grocery stores. Use only the lower white part of the stalk. 3. Substitute bok choy with cabbage, Napa cabbage or broccoli. You can use either fresh or dried shiitake mushrooms.