In a large saucepan, bring broth to a BOIL; ADD lemon grass, pepper flakes, lemon juice and fish sauce; BOIL for five minutes.
REDUCE heat; REMOVE lemon grass pieces.
ADD shrimp; COOK until shrimp are opaque and pink, about three to four minutes.
ADD green onions and cilantro; SERVE at once.
To prepare lemon grass, trim away bottom and top, leaving a section about three to four inches long. Remove dry outer husk. Slice in half lengthwise. If lemongrass is not available, substitute the zest of a half lemon.
Per Serving (excluding unknown items): 158 Calories; 3g Fat (17.9% calories from fat); 22g Protein; 9g Carbohydrate; 1g Dietary Fiber; 115mg Cholesterol; 1079mg Sodium. Exchanges: 0 Grain(Starch); 2 1/2 Lean Meat; 0 Vegetable; 0 Fruit.