**Core Concept**
Congenital arteriovenous (A-V) fistulas are abnormal connections between arteries and veins, leading to increased blood flow and pressure in the affected limb. This can result in various clinical manifestations, including limb hypertrophy, varicose veins, and potential cardiac complications. The thigh is a common site for A-V fistulas due to its rich vascular supply.

**Why the Correct Answer is Right**
The presence of congenital A-V fistulas in the thigh can lead to increased blood flow and pressure in the affected limb, causing hypertrophy of the muscles and soft tissues. This is due to the direct shunting of oxygenated blood from the arterial system to the venous system, bypassing the capillary bed. As a result, the affected limb may exhibit signs of increased blood flow, including warmth, swelling, and erythema.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because congenital A-V fistulas in the thigh can indeed lead to limb hypertrophy, making this a correct association.

**Option B:** This option is incorrect because varicose veins are a common complication of congenital A-V fistulas, especially in the lower extremities.

**Option C:** This option is incorrect because cardiac complications, such as heart failure or pulmonary hypertension, can occur in advanced cases of congenital A-V fistulas due to increased blood flow and pressure.

**Clinical Pearl / High-Yield Fact**
The "triple A" sign on physical examination (arterial thrill, audible bruit, and aneurysm) is highly suggestive of a congenital A-V fistula.

**Correct Answer: D**