A 23-year-old male injured in an industrial explosion was found to have multiple small metal fragments in his thoracic cavity. Since the pericardium was torn inferiorly, the surgeon began to explore for fragments in the pericardial sac. Slipping her hand under the heart apex, she slid her fingers upward and to the right within the sac until they were stopped by the cul-de-sac formed by the pericardial reflection near the base of the heart. Her fingertips were then in the: Coronary sinus Coronary sulcus Costomediastinal recess Oblique sinus Transverse sinus

The surgeon's fingertips were then in the Oblique sinus.

Here's the step-by-step explanation:

1. The pericardial sac is the cavity that contains the heart. It has two parts: the fibrous pericardium (the tough outer layer) and the serous pericardium (the thin inner layer).

2. The serous pericardium is further divided into two layers: the parietal layer (which lines the fibrous pericardium) and the visceral layer (which is attached to the heart muscle). Between these two layers is the pericardial cavity, which contains a small amount of fluid for lubrication.

3. The pericardial sac has two recesses or sinuses formed by reflections of the serous pericardium: the transverse sinus and the oblique sinus.

4. The transverse sinus is a horizontal passage behind the aorta and pulmonary trunk and in front of the superior vena cava. It is not accessible by the surgeon's hand during surgery.

5. The oblique sinus is a vertical passage located behind the heart. It is formed by the left and right pulmonary veins entering the heart, and it is bounded by the pericardial reflection near the base of the heart. This is the cul-de-sac that the surgeon's fingers reached.

Therefore, the surgeon's fingertips were in the oblique sinus.