A 6-month-old infant presents with recurrent infections and failure to thrive. Laboratory investigations reveal a deficiency of adenosine deaminase (ADA). Which of the following immunodeficiency disorders is most likely associated?

Correct Answer: C. Severe Combined Immunodeficiency (SCID)

Adenosine deaminase (ADA) deficiency is a well-established cause of autosomal recessive SCID, accounting for approximately 10–15% of all SCID cases in India. ADA catalyzes the deamination of adenosine and deoxyadenosine to inosine and deoxyinosine. When ADA is deficient, toxic metabolites (deoxyadenosine and deoxyadenosine triphosphate) accumulate, particularly in lymphocytes, which have high levels of deoxycytidine kinase. These metabolites are highly toxic to T cells and B cells, causing apoptosis and severe lymphoid hypoplasia. The result is profound deficiency of both T and B cell immunity (hence "combined"), along with variable NK cell dysfunction. Clinically, infants present within the first 6 months with recurrent infections (bacterial, viral, fungal, and opportunistic), failure to thrive, and hepatosplenomegaly. The diagnosis is confirmed by elevated plasma and urine deoxyadenosine levels and absent or very low ADA enzyme activity. This is distinct from other immunodeficiencies because the metabolic defect directly causes lymphocyte death, making SCID the most likely diagnosis in an ADA-deficient infant with this clinical presentation.

Why the other options are wrong

A. Hypogammaglobulinemia — Hypogammaglobulinemia (e.g., X-linked agammaglobulinemia) is a primary B cell deficiency with relatively preserved T cell immunity. Patients typically present after 6 months of age with recurrent bacterial infections but normal viral and fungal immunity. ADA deficiency causes combined T and B cell deficiency with early presentation and opportunistic infections—a hallmark of SCID, not isolated hypogammaglobulinemia. B. Wiskott-Aldrich Syndrome — Wiskott-Aldrich Syndrome is an X-linked disorder presenting with the triad of thrombocytopenia (low platelets), eczema, and recurrent infections. It results from defective actin cytoskeleton organization, not a metabolic enzyme deficiency. While it causes immunodeficiency, the clinical presentation includes bleeding manifestations and eczema, which are absent in ADA deficiency. The pathophysiology is entirely different. D. DiGeorge Syndrome — DiGeorge Syndrome (22q11 deletion) is characterized by thymic hypoplasia/aplasia leading to T cell deficiency, with variable B cell involvement. It is a developmental disorder, not a metabolic enzyme deficiency. Patients often have cardiac defects, cleft palate, and hypocalcemia. ADA deficiency is a biochemical defect causing toxic metabolite accumulation—a fundamentally different pathophysiology.

High-Yield Facts

Mnemonics

SCID Metabolic Causes (ADA & PNP) ADA → Adenosine Deaminase deficiency (10–15% SCID); PNP → Purine Nucleoside Phosphorylase deficiency (rare). Both cause toxic metabolite accumulation in lymphocytes → apoptosis → combined T+B cell loss. ADA Toxicity = Lymphocyte Death Deoxyadenosine accumulates → phosphorylated to deoxyadenosine triphosphate (dATP) → inhibits ribonucleotide reductase → blocks dNTP synthesis → apoptosis in T and B cells.

NBE Trap

NBE may pair ADA deficiency with hypogammaglobulinemia to trap students who focus only on "B cell deficiency" without recognizing that ADA causes combined T+B deficiency with early presentation and opportunistic infections—the hallmark of SCID, not isolated antibody deficiency.

Clinical Pearl

In Indian pediatric practice, any 6-month-old with failure to thrive, recurrent infections including PCP or candidiasis, and a metabolic enzyme defect should raise suspicion for SCID. ADA-SCID is treatable with enzyme replacement (pegadenosine) or curative HSCT, making early diagnosis critical for referral to tertiary immunology centers.

_Reference: OP Ghai Pediatrics Ch. 11 (Immunodeficiency Disorders); Harrison Ch. 375 (Primary Immunodeficiency Diseases)_