A 52-year-old woman presents with progressively worsening fa…

A 52-yeаr-оld wоmаn presents with prоgressively worsening fаtigue over the past six months. She reports becoming short of breath when climbing one flight of stairs and has noticed brittle fingernails and increasing difficulty completing her daily walks. She has a history of heavy menstrual bleeding but has not seen a gynecologist in several years.Physical examination reveals conjunctival pallor. Cardiac examination demonstrates a soft systolic flow murmur. No lymphadenopathy or splenomegaly is noted.Laboratory results are:Hemoglobin 8.8 g/dLHematocrit 28%MCV 71 fLMCH 22 pgRDW 19%Platelets 415,000/mm³Which additional laboratory study would best confirm the suspected diagnosis?