**Core Concept**
Mixed connective-tissue disease (MCTD) is a chronic autoimmune disorder characterized by overlapping features of lupus, scleroderma, and rheumatoid arthritis. It is marked by the presence of high levels of autoantibodies, particularly anti-U1 RNP antibodies.

**Why the Correct Answer is Right**
Hypogammaglobulinemia, a condition of low gamma globulin levels, is not characteristic of MCTD. In contrast, patients with MCTD often exhibit hypergammaglobulinemia, characterized by elevated levels of gamma globulins. This is due to the polyclonal activation of B cells resulting from the autoimmune nature of the disease.

**Why Each Wrong Option is Incorrect**
**Option B:** Membranous glomerulonephritis is a possible renal manifestation of MCTD, often associated with the presence of anti-U1 RNP antibodies. It is not an exception to the characteristics of MCTD.
**Option C:** Polyarthritis is a common feature of MCTD, presenting as a symmetric, non-erosive arthritis that may involve multiple joints. This symptom is consistent with the disease's clinical presentation.
**Option D:** CNS involvement, including headaches, seizures, and psychosis, can occur in MCTD patients due to the presence of autoantibodies and vasculitis.

**Clinical Pearl / High-Yield Fact**
MCTD often presents with a combination of joint pain, skin thickening, and lung involvement, making it essential to consider a comprehensive diagnostic workup, including serologic tests for anti-U1 RNP antibodies.

**✓ Correct Answer: A. Hypogammaglobulinemia**