**Core Concept**
Hyperthyroidism is a condition characterized by an overproduction of thyroid hormones, leading to increased metabolic rate, cardiac stimulation, and potential cardiopulmonary manifestations. The primary cardiac effect is an increase in heart rate and contractility.

**Why the Correct Answer is Right**
The correct answer, **C. ac**, reflects the cardiopulmonary manifestations of hyperthyroidism. **Tachycardia (a)** is a direct result of the increased sympathetic tone and heightened metabolic rate associated with hyperthyroidism. **Cardiomegaly (c)** occurs due to the increased workload on the heart, leading to hypertrophy and dilation. This is a consequence of the sustained increased contractility and heart rate.

**Why Each Wrong Option is Incorrect**
**Option B:** Sinus bradycardia is incorrect because hyperthyroidism typically leads to an increased heart rate, not a decreased one. The sympathetic stimulation associated with hyperthyroidism would not cause a decrease in heart rate.

**Option D:** Low output cardiac failure is incorrect because hyperthyroidism initially causes an increase in cardiac output, not a decrease. The increased contractility and heart rate lead to an initial increase in cardiac output, which can eventually lead to cardiomegaly and potentially heart failure, but this is not the primary manifestation.

**Clinical Pearl / High-Yield Fact**
In hyperthyroidism, the increased metabolic rate and cardiac stimulation can lead to a widened pulse pressure due to increased stroke volume and decreased peripheral resistance. This is an important clinical correlation to remember when evaluating patients with hyperthyroidism.

**Correct Answer:**
✓ Correct Answer: C. ac