Restore distractor-specific follow-up questions

api/QuestionsApi/Services/DynamicQuestionGenerator.cs

@@ -283,7 +283,8 @@ public sealed class DynamicQuestionGenerator
         IReadOnlyDictionary<string, string> values)
     {
         var result = new Dictionary<string, WrongAnswerFollowup>();
-        foreach (var distractor in distractors)
+        var distractorList = distractors.ToList();
+        foreach (var distractor in distractorList)
         {
             if (templates.TryGetValue(distractor.Id, out var template))
             {
@@ -297,16 +298,18 @@ public sealed class DynamicQuestionGenerator
                 };
                 continue;
             }
+            var alternatives = distractorList.Where(value => value.Id != distractor.Id).Take(2).ToArray();
             result[distractor.Id] = new WrongAnswerFollowup
             {
-                Prompt = "Which option best addresses the decisive finding in this case?",
+                Prompt = $"Which limitation of option {distractor.Id} should have ruled it out in this case?",
                 Choices =
                 [
-                    new QuestionChoice { Id = correct.Id, Text = Render(correct.TextTemplate, values) },
-                    new QuestionChoice { Id = distractor.Id, Text = Render(distractor.TextTemplate, values) }
+                    new QuestionChoice { Id = "A", Text = Render(distractor.ExplanationTemplate, values) },
+                    new QuestionChoice { Id = "B", Text = Render(alternatives[0].ExplanationTemplate, values) },
+                    new QuestionChoice { Id = "C", Text = Render(alternatives[1].ExplanationTemplate, values) }
                 ],
-                CorrectChoiceId = correct.Id,
-                TeachingPoint = $"{Render(distractor.ExplanationTemplate, values)} {Render(correct.ExplanationTemplate, values)}"
+                CorrectChoiceId = "A",
+                TeachingPoint = Render(distractor.ExplanationTemplate, values)
             };
         }
         return result;

api/QuestionsApi/Services/QuestionValidator.cs

@@ -49,6 +49,21 @@ public static class QuestionValidator
             if (!entry.Value.Choices.Any(choice => choice.Id == entry.Value.CorrectChoiceId))
                 return "wrongAnswerFollowups correctChoiceId must match a follow-up choice id";
         }
+        if (req.IsPublished)
+        {
+            if (req.WrongAnswerFollowups.Any(entry => entry.Value.Choices.Count != 3))
+                return "published wrongAnswerFollowups require exactly three choices per distractor";
+            if (!AllDistinct(req.WrongAnswerFollowups.Values.Select(value => value.Prompt)))
+                return "published wrongAnswerFollowups require a different prompt for every distractor";
+            if (!AllDistinct(req.WrongAnswerFollowups.Values.Select(value => value.TeachingPoint)))
+                return "published wrongAnswerFollowups require a different teachingPoint for every distractor";
+            if (!AllDistinct(req.WrongAnswerFollowups.Values.Select(value =>
+                    value.Choices.Single(choice => choice.Id == value.CorrectChoiceId).Text)))
+                return "published wrongAnswerFollowups require a different correct follow-up answer for every distractor";
+            if (!AllDistinct(req.WrongAnswerFollowups.Values.Select(value => string.Join("|",
+                    value.Choices.Select(choice => Normalize(choice.Text)).OrderBy(text => text, StringComparer.Ordinal)))))
+                return "published wrongAnswerFollowups require a different choice set for every distractor";
+        }
 
         if (req.ReviewFollowups is null) return "reviewFollowups must be an array";
         if (req.ReviewFollowups.Count > 10) return "questions support at most ten review follow-ups";
@@ -75,6 +90,16 @@ public static class QuestionValidator
         return null;
     }
 
+    private static bool AllDistinct(IEnumerable<string> values)
+    {
+        var normalized = values.Select(Normalize).ToList();
+        return normalized.All(value => value.Length > 0)
+            && normalized.Distinct(StringComparer.Ordinal).Count() == normalized.Count;
+    }
+
+    private static string Normalize(string value) => string.Join(' ', value.Trim().ToLowerInvariant()
+        .Split((char[]?)null, StringSplitOptions.RemoveEmptyEntries));
+
     public static string? ValidateLabs(IReadOnlyCollection<QuestionLab>? labs, bool requireComplete)
     {
         if (labs is null)