A 20-year-old male is seen by his physician following a week…

A 20-year-old male is seen by his physician following a weekend backpacking trip to the mountains.  His CBC reveals a peripheral blood morphology including hypochromia, anisocytosis, and poikilocytosis.  Cellulose acetate hemoglobin electrophoresis yields Hgb A = 47%, Hgb S = 50%, Hgb A2 = 3%.  These results are consistent with which of the following: