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Author Ahmed Zaghw, M.D. [1]
Exam Type USMLE Step 2 CK
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Prompt A 18-year-old African american male comes to the office for the evaluation of pain in his right hip that started 7 weeks ago. The pain has gradually progressed, and now it limits his daily activities. He has sickle cell disease and was hospitalized 4 months ago due to a painful crisis that was successfully treated with hydration, oxygen, and analgesics. His temperature is 37.4C (99F), blood pressure is 120/90 mmHg, pulse is 90/min, and respirations are 14/min. Physical examination reveals no local tenderness, but there is restriction of abduction and internal rotation of the hip. What is the most likely diagnosis?
Answer A Femoral fracture
Answer A Explanation Incorrect- Femoral fracture is not commonly associated with sickle cell anemia.
Answer B Joint effusion from septic arthritis
Answer B Explanation Incorrect- Joint effusion from septic arthritis could occur as a complications of sickle cell anemia. Joint involvement is suspected with limitation of movement. However, absence of fever could exclude the infectious etiology.
Answer C Avascular necrosis
Answer C Explanation Correct- Avascular necrosis is one of the complications of the sickle cell anemia. It occurs gradually by multiple attacks of vasoocclusive insults in the head of the femur. The joint mobility progressively deteriorate over time, but not associated with fever.
Answer D Legg–Calvé–Perthes syndrome
Answer D Explanation Incorrect- Legg-Calvé-Perthes syndrome is a idiopathic degenerative disease of the hip joint, where a loss of bone mass leads to some degree of collapse of the hip joint, that is, to deformity of the ball of the femur and the surface of the hip socket. The disease is typically found in young children and small dogs, and it can lead to osteoarthritis in adults.
Answer E Osteomyelitis caused by Staphylococcus
Answer E Explanation Incorrect- Absence of fever excludes the infectious etiology, however staphylococcal bone infection is common with sickle cell disease.
Right Answer C
Explanation Sickle-cell disease is a group of genetic disorders caused by mutation in the β-globin chain gene of hemoglobin at the 6th position replacing glutamic acid to valine. HbS polymerizes reversibly when deoxygenated to form a network of fibrous hemoglobin polymers that stiffens the RBC membrane, giving it a sickle shape. These sickled cells loose the pliability to cross thin capillaries and possess a sticky membrane, giving it a property to adhere to the endothelium of blood vessels, thereby causing vaso-occlusion. It causes significant morbidity and mortality, particularly in people in the Mediterranean and African region.

A vaso-occlusive crisis is caused by sickle-shaped red blood cells that obstruct capillaries and restrict blood flow to an organ, resulting in ischemia, pain, and organ damage.

Because of its narrow vessels and function in clearing defective red blood cells, the spleen is frequently affected. It is usually infarcted before the end of childhood in individuals suffering from sickle-cell anemia. This autosplenectomy increases the risk of infection from encapsulated organisms. Preventive antibiotics and vaccinations are recommended for those with such asplenia.

Bones, especially weight-bearing bones, are also a common target of vaso-occlusive damage. This is due to bone ischemia.

A recognised type of sickle crisis is the acute chest crisis, a condition characterised by fever, chest pain, hard breathing, and pulmonary infiltrate on chest X-ray. Given that pneumonia and intrapulmonary sickling can both produce these symptoms, the patient is treated for both conditions.
Educational Objective: Avascular necrosis should be considered in patients with multiple attacks of sickle cell crisis.
References: J Infect Dis. 1977 Aug;136 Suppl:S25-30, Sickle cell anemia and severe infections due to encapsulated bacteria.(http://www.ncbi.nlm.nih.gov/pubmed/330779)

Pediatr Blood Cancer. 2010 Sep;55(3):401-6. , Susceptibility to invasive bacterial infections in children with sickle cell disease.(http://www.ncbi.nlm.nih.gov/pubmed/20232448)

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