<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 65-year-old woman, with end-stage renal disease ESRD on hemodialysis, presents with pain, swelling and discoloration of her right leg and foot. She says that she started twice-weekly hemodialysis 2 weeks ago and has had no issues until 1 week ago when she noticed a warm, painful swelling of the back of her right leg and right foot after finishing her dialysis session. Over the week, she says these symptoms have steadily worsened and, in the last few days, her right foot has become discolored. Past medical history is significant for ESRD secondary to long-standing hypertension. Current medications are verapamil 200 mg orally daily and unfractionated heparin that is given during hemodialysis. Her vital signs include: temperature 37.0C 98.6F, blood pressure 14575 mm Hg, pulse 88min, respirations 15min, and oxygen saturation 99 on room air. On physical examination, the patient is alert and cooperative. The cardiac exam is normal. Lungs are clear to auscultation. The abdomen is soft and nontender with no hepatosplenomegaly. The right calf is swollen, warm, and erythematous. Physical findings of the patients right foot are shown in the exhibit. Laboratory findings are significant for the following:Sodium 141 mEqLPotassium 4.9 mEqLChloride 104 mEqLBicarbonate 25 mEqLBUN 32 mgdLCreatinine 3.1 mgdLGlucose fasting 75 mgdL Bilirubin, conjugated 0.5 mgdL Bilirubin, total 1.0 mgdLAST SGOT 22 UL ALT SGPT 23 UL Alkaline phosphatase 56 UL Bleeding time 19 min Prothrombin time PT 11 s Partial thromboplastin time PTT 30 s WBC 8,500mm3 RBC 4.10 x 106mm3Hematocrit 41.5Hemoglobin 13.5 gdL Platelet count 100,000mm3 previously 200,000mm3Which of the following is the next best diagnostic step in this patient?? A: HeparinPF4 enzyme-linked immunosorbent assay ELISA, B: Functional assay for factor VIII, C: Serotonin release assay, D: Peripheral blood smear, E: Flow cytometry for CD55,  <response>: This is a patient with end-stage renal disease on hemodialysis who is presenting with signs and symptoms of thrombosis in her right lower extremity, including pain, swelling, erythema, and skin discoloration. The description and photo indicate ischemia and likely thrombosis of veins andor arteries in the right leg. Key findings in the history include:- Onset of symptoms 1 week after starting hemodialysis- Symptoms worsened after dialysis sessions- Taking unfractionated heparin during dialysisKey findings on exam:- Swelling, warmth, erythema of right calf - Ischemic changes to right footRelevant lab abnormalities:- Thrombocytopenia - Prolonged PTTThe main diagnostic considerations are:- Heparin-induced thrombocytopenia HIT- Arterial thrombosis or venous thrombosis- VasculitisTo confirm HIT, the next best diagnostic test would be a heparinPF4 antibody assay. This is the most sensitive and specific test for HIT. The other answer choices are not useful for diagnosing HIT.A: HeparinPF4 enzyme-linked immunosorbent assay ELISA 