Which of the following can be seen in SCID?
**Core Concept**
Severe Combined Immunodeficiency (SCID) is a rare genetic disorder characterized by a profound deficiency in both T-cell and B-cell immunity. This results in an increased susceptibility to infections, particularly those caused by opportunistic pathogens. Patients with SCID often present with recurrent and severe infections, failure to thrive, and a significantly impaired immune response.
**Why the Correct Answer is Right**
SCID is caused by mutations in various genes involved in the development and function of immune cells. These genes include those encoding for the common gamma chain of the IL-2 receptor (IL2RG), adenosine deaminase (ADA), and the recombination activating genes 1 and 2 (RAG1 and RAG2), among others. The absence or malfunction of these proteins leads to an impaired ability of immune cells to develop and function properly, resulting in the clinical manifestations of SCID.
**Why Each Wrong Option is Incorrect**
**Option A:** While eczema and recurrent infections can be seen in SCID, they are not pathognomonic and can also be seen in other conditions such as atopic dermatitis and common variable immunodeficiency.
**Option B:** Failure to thrive is indeed a characteristic of SCID, but it is not specific to this condition and can be seen in various other diseases, including malignancies and metabolic disorders.
**Option C:** While opportunistic infections are a hallmark of SCID, they can also be seen in other immunocompromised states, such as HIV/AIDS and post-transplant patients.
**Clinical Pearl / High-Yield Fact**
A key diagnostic clue for SCID is a low or absent T-cell count on flow cytometry, particularly in the context of recurrent and severe infections.
**Correct Answer:** D.