Review Question - QID 216383

A 24-year-old G1P0 pregnant woman presents for her first prenatal visit with a new physician. She states she was "always tired" prior to pregnancy but has had worsening fatigue for the past 5 weeks. She does not endorse any other concerns. She has no recent infections or rashes. She had vaginal spotting in her first trimester and has no other history of recent trauma, vaginal, or rectal bleeding. She takes no medications and has no history of surgery or blood transfusions. She eats a balanced diet including red meat, fish, grains, and leafy green vegetables. She immigrated from Thailand 3 weeks ago and has been unable to obtain her medical records. Her temperature is 37.0°F (98.6°C), blood pressure is 110/70 mmHg, pulse is 88/min, and respirations are 16/min. Cardiovascular, pulmonary, and skin exams are unremarkable. An abdominal exam demonstrates a fundus palpable at the height of the umbilicus. Ultrasound examination demonstrates an intrauterine pregnancy at 22 weeks gestational age and a posterior placenta. Laboratory results are as follows:

Hemoglobin concentration: 8.0 g/dL
Mean corpuscular hemoglobin concentration: 28%
Mean corpuscular volume: 69 µm^3
Red blood cell distribution width: 12.8% (12.2-16.1%)

Which of the following is the most likely cause of this patient's anemia?