### **Official Solution & Concept Analysis (GATE BT 2023 Q.23)**
**1. Governing Concept**:
- **Topic**: Hybridoma Technology & Monoclonal Antibody Selection
- In hybridoma technology (Köhler and Milstein), fusion of splenocytes (B-cells) with myeloma cells creates immortal antibody-secreting hybridomas.
- Selection occurs in **HAT medium** (Hypoxanthine, Aminopterin, Thymidine):
- Aminopterin blocks the de novo nucleotide synthesis pathway.
- Cells must utilize the salvage pathway using HGPRT (**Hypoxanthine-guanine phosphoribosyltransferase**) and Thymidine Kinase (TK).
- The parental myeloma cell line is deliberately engineered to be **HGPRT-deficient ($\text{HGPRT}^-$)**. Consequently, unfused myeloma cells die in HAT medium.
**2. Option Analysis**:
- **Option (A) [Hypoxanthine-guanine phosphoribosyltransferase] is CORRECT**: The specific purine salvage enzyme lacking in mutant myeloma fusion partners.
- **Option (B) [Alanine aminotransferase] is INCORRECT**: Liver transaminase unrelated to nucleotide salvage.
- **Option (C) [Triose phosphate isomerase] is INCORRECT**: Essential glycolytic enzyme present in all cells.
- **Option (D) [Glycosyltransferase] is INCORRECT**: Golgi enzyme involved in protein glycosylation.
**3. Final Answer**:
- **Correct Choice**: **A**
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