USMLE Herditary Spherocytosis Normocytic with Increased MCHC?

[Cleos Half]

I just did a question on USMLE-Rx re: Hereditary Spherocytosis. I got the diagnosis right but in the answer explanation, it says that it is a normocytic anemia but at the same time, there is an increased mean corpuscular hemoglobin concentration. How is this possible when you are normocytic (MCV: 90) and anemic (Hb: 11.5)?

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[placeboconspiracy]

I believe there is extravascular hemolysis and premature removal of the RBCs by the spleen leading to a lower amount of RBCs available in the circulation. Whatever hemoglobin is available, gets packed into the small, round RBCs that lack central pallor.

 

[AnatomyGrey12]

Increased MCHC is due to lack of GIP on the surface of the RBC. The anemia is because the spleen filters the spherocytes out of circulation (extravascualr hemolysis). That’s why the “cure” for it is splenectomy. It doesn’t cure the underlying disease but it fixes the anemia.