A patient of African ethnicity shows marked target cells and large oblong crystalloid structures in her red cells. What is her most likely condition?

The presence of marked target cells along with large oblong crystalloid structures in red blood cells is indicative of Hb C disease. In Hb C disease, there is a mutation that causes the hemoglobin to polymerize under conditions of low oxygen tension, leading to the formation of these characteristic crystalloid structures within the red cells.

Target cells, which are red blood cells with a darker center and lighter peripheral zone, are often seen in various hemoglobinopathies, but in the context of Hb C disease, the presence of the oblong crystalloid structures is particularly diagnostic. The distinct morphology is the result of the alteration in hemoglobin as a result of the single amino acid substitution in the beta chain of hemoglobin.

While Hb C trait would show some target cells, it typically does not exhibit the same degree of red cell pathology as seen in Hb C disease. Hb SC disease would present with a different morphology due to the combination of hemoglobins S and C, while Hb SS disease is characterized by an entirely different set of red cell abnormalities due to the presence of hemoglobin S.