Conditional Linq Select on Nested object

Given I have a simple object like this

public class TestA
{
    public int TestAId { get; set; }
    public string Description { get; set; }

    public IEnumerable<TestB> TestBCollection { get; set; }
}

public class TestB
{
    public int TestBId { get; set; }
    public int FkTestAId { get; set; }
    public string Description { get; set; }
}

List<TestA> a = new List<TestA>()
            {
                new TestA()
                {
                    TestAId = 1,
                    Description = "Test A Description",
                    TestBCollection = new List<TestB>()
                    {
                        new TestB()
                        {
                            TestBId = 10,
                            FkTestAId = 1,
                            Description = "Test B Description" // this must be used because of the matching FK
                        }
                    }
                }
            };

I am trying to Select the description property on TestA but if there is a value in TestB where TestAId == FkTestAId I want to select TestB Description