**Core Concept**
The question pertains to the diagnosis of a congenital adrenal hyperplasia (CAH) in a newborn, specifically focusing on the enzymatic deficiency responsible for the condition.

**Why the Correct Answer is Right**
CAH is caused by mutations in genes encoding enzymes of the steroidogenesis pathway, leading to an accumulation of steroid precursors. 21-Hydroxylase deficiency is the most common cause, resulting from mutations in the CYP21A2 gene. This deficiency impairs the conversion of 17-hydroxyprogesterone (17-OHP) to 11-deoxycortisol, leading to an overproduction of androgens and an underproduction of cortisol and aldosterone.

**Why Each Wrong Option is Incorrect**
**Option A:**
17-Hydroxylase deficiency is a rare cause of CAH, resulting from mutations in the CYP17A1 gene. It leads to an underproduction of cortisol and androgens, but an overproduction of mineralocorticoids, which is not consistent with the presentation of CAH.

**Option B:**
3-Beta-Hydroxysteroid dehydrogenase (3β-HSD) deficiency is a cause of CAH, but it affects the conversion of pregnenolone to progesterone and 17-hydroxypregnenolone to 17-hydroxyprogesterone. This deficiency leads to an underproduction of all classes of steroids.

**Option C:**
11-Beta-Hydroxylase deficiency is another rare cause of CAH, resulting from mutations in the CYP11B1 gene. It leads to an overproduction of 11-deoxycortisol, which has mineralocorticoid activity, causing hypertension.

**Option D:**
Aldosterone synthase deficiency is a rare cause of CAH, resulting from mutations in the CYP11B2 gene. It leads to an underproduction of aldosterone, causing hypotension and hyperkalemia.

**Clinical Pearl / High-Yield Fact**
The classic presentation of CAH includes ambiguous genitalia in female infants, virilization in male infants, and an elevated level of 17-hydroxyprogesterone (17-OHP) in the neonatal screening test.

**Correct Answer: A. 17-Hydroxylase deficiency is a rare cause of congenital adrenal hyperplasia.