Spectral efficiency tells us how much information we can fit into a given amount of spectrum. It is defined as the data rate (bits per second) divided by the bandwidth (hertz), giving units of bits per second per hertz. This captures the trade-off between sending more data and using the same frequency range. The statement that the ratio of data rate to bandwidth is the spectral efficiency fits this definition exactly, so it’s the best answer. The inverse of the modulation order isn’t correct because modulation order describes how many distinct symbols are used, and spectral efficiency depends on how many bits per symbol (log2 of the order) and how much bandwidth is required, not simply the inverse of the order. The module used for error correction relates to coding, which affects reliability and sometimes effective data rate, but not the fundamental definition of spectral efficiency. The energy-per-bit to noise ratio is a measure of energy efficiency and link quality, not how efficiently spectrum is used.

Einthoven's triangle is the geometric representation of the three standard bipolar limb leads around the heart. It’s formed by the potentials measured between pairs of limb electrodes: right arm to left arm, right arm to left leg, and left arm to left leg. These leads project the heart’s electrical dipole onto three axes spaced roughly 60 degrees apart, creating a triangle that helps describe the heart’s electrical activity in the frontal plane. Chest leads (precordial) sit on the chest and are unipolar; they map the heart in a different plane and are not part of this triangle. The right leg electrode is used as a reference/ground and isn’t one of the triangle’s vertices. So the triangle around the heart formed by the bipolar limb leads is the concept described.

Time intervals on ECG paper are read along the horizontal axis. A small box represents 0.04 seconds, and five adjacent small boxes make one large box. Multiplying 0.04 by five gives 0.2 seconds, so a large box lasts 0.2 seconds. This aligns with the standard paper speed of 25 mm per second, where one second spans five large boxes. This quick reference helps you estimate intervals and heart rate directly from the tracing.

The impulse in normal sinus rhythm begins with the Sinoatrial node, the heart’s natural pacemaker. Located in the right atrium near the superior vena cava, SA node cells automatically generate impulses, setting the pace for the heart usually around 60–100 beats per minute. This impulse starts there and then travels through the atria to cause atrial contraction, proceeds to the atrioventricular node (which provides a brief delay to allow filling), and then moves down the His-Purkinje system to rapidly depolarize the ventricles. Purkinje fibers and bundle branches are key conduits for the signal to reach the ventricles, but they do not initiate the rhythm themselves. The AV node can take over only if the SA node fails, which is not the case in normal sinus rhythm.