I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
A 50-year-old man comes to the office because of a 2-month history of increasing daytime somnolence. He has obstructive sleep apnea for which he has only intermittently used a continuous positive airway pressure device. He is 170 cm (5 ft 7 in) tall and weighs 181 kg (400 lb.); BMI is 63 kg/m2. His temperature is 37°C (98.6°F), pulse is 100/min, respirations are 12/min, and blood pressure is 135/80 mm Hg. Physical examination shows a gray-blue tinge to the lips, earlobes, and nail beds. Cardiac examination shows no other abnormalities. Arterial blood gas analysis on room airshows a pH of 7.31, PCO2 of 70 mm Hg, and PO2 of 50 mm Hg.
Which of the following additional findings would be most likely in this patient?
(A) Decreased serum bicarbonate concentration
(B) Increased hemoglobin concentration
(C) Increased total lung capacity
(D) Left ventricular hypertrophy
Copyright © 1996-2021 Federation of State Medical Boards (FSMB) and National Board of Medical Examiners (NBME). All rights reserved. The United States Medical Licensing Examination® (USMLE®) is a joint program of the FSMB and NBME.
Answer
This patient’s presentation reveals severe obesity, signs of cyanosis, and respiratory acidosis leading to chronic hypoventilation.
In severe obesity hypoventilation syndrome (OHS), chronic hypoventilation leads to elevated levels of carbon dioxide (PCO2), decreased levels of oxygen (PO2) in the blood and increased serum bicarbonate concentration.
Chronic hypoxemia stimulates erythropoiesis, leading to an increase in the production of red blood cells and subsequently increased hemoglobin concentration. This physiological response is commonly observed in conditions associated with chronic hypoxemia, such as OHS.
Therefore, the most likely additional finding in this patient would be increased hemoglobin concentration.
So, the correct answer is (B) Increased hemoglobin concentration.
More Repeated USMLE Questions Step 1
Check this Review Book for Step 1