## Core Concept
The patient's presentation of spidery veins, also known as **telangiectasias** or **spider nevi**, on her face, upper chest, and arms during the third trimester of pregnancy is a common dermatological manifestation associated with pregnancy. These lesions are typically benign but can be a source of concern for the patient.

## Why the Correct Answer is Right
The appearance of spider nevi during pregnancy is attributed to the increased levels of **estrogen**. This condition is often associated with **vascular changes** and can be seen in conjunction with other vascular changes such as palmar erythema and spider angiomas. Given that these lesions are usually benign and related to hormonal changes during pregnancy, the most appropriate next step would be to **reassure the patient**. Most spider nevi resolve spontaneously after pregnancy.

## Why Each Wrong Option is Incorrect
**Option A:** Suggesting a biopsy might be an overly aggressive approach for lesions that are typically benign and associated with pregnancy. There's no indication that the lesions are suspicious for malignancy based on the description provided.

**Option B:** While monitoring the patient's condition is important, there's no specific indication for a detailed **vascular workup** without other symptoms suggesting vascular disease.

**Option C:** Although certain cosmetic treatments can be used for spider nevi, recommending **sclerotherapy** or laser treatment during pregnancy might not be the best approach due to potential risks and the fact that these lesions often resolve postpartum.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **spider nevi** are common during pregnancy due to **estrogenic effects** on blood vessels. They typically do not require treatment and **resolve on their own** after delivery. Reassurance is a crucial part of managing these patients.

## Correct Answer Line
**Correct Answer: D. Reassure the patient.**